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cik0000225604:MFSMunicipalHighIncomeFundBlendedIndex38Member 2016-01-01 2025-12-31 iso4217:USD pure N-1A MFS® SERIES TRUST III 2026-05-29 2026-05-29 MFS® Global High Yield Fund Summary of Key Information MHOAX MHOBX MHOCX MHOIX MHORX MHOSX MHOTX MHOUX MHOVX Investment Objective The fund’s investment objective is to seek total return with an emphasis on high current income, but also considering capital appreciation. Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 12 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. 100000 Shareholder Fees (fees paid directly from your investment): 0.0425 0 0 0 0 0 0 0 0 0.0100 0.0400 0.0100 0 0 0 0 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0065 0.0065 0.0065 0.0065 0.0065 0.0065 0.0065 0.0065 0.0065 0.0025 0.0100 0.0100 0 0.0100 0.0050 0.0025 0 0 0.0035 0.0035 0.0035 0.0035 0.0035 0.0035 0.0035 0.0035 0.0025 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0002 0.0127 0.0202 0.0202 0.0102 0.0202 0.0152 0.0127 0.0102 0.0092 -0.0025 -0.0025 -0.0025 -0.0025 -0.0025 -0.0025 -0.0025 -0.0025 -0.0023 0.0102 0.0177 0.0177 0.0077 0.0177 0.0127 0.0102 0.0077 0.0069 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 525 787 1069 1873 580 909 1265 2135 180 609 1065 2135 280 609 1065 2135 180 609 1065 2135 79 300 539 1225 180 609 1065 2328 129 456 805 1791 104 378 673 1512 79 300 539 1225 70 270 487 1110 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 38% of the average value of its portfolio. 0.38 Principal Investment Strategies MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests directly and/or indirectly through investment in mutual funds advised by MFS at least 80% of the fund’s net assets in high income debt instruments. Debt instruments include corporate debt instruments, foreign government securities, floating rate loans, and other obligations to repay money borrowed. High income debt instruments are debt instruments that are below investment grade quality. Below investment grade quality debt instruments are debt instruments rated BB+ or lower by S&P or Fitch, or Ba1 or lower by Moody’s, or comparably rated by another Nationally Recognized Statistical Rating Organization ("NRSRO"). MFS may invest the fund’s assets in other types of debt instruments. MFS may invest up to 100% of the fund’s assets in below investment grade quality debt instruments.MFS invests the fund’s assets in U.S. and foreign securities, including emerging market securities. MFS normally invests the fund's assets across different industries, sectors, countries, and regions, but MFS may invest a significant percentage of the fund’s assets in issuers in a single industry, sector, country, or region. MFS generally invests the fund's assets in at least three different countries and invests a percentage of the fund's net assets in securities of foreign issuers equal to at least the lesser of 40% or the percentage of foreign issuers in the ICE BofA Global High Yield - Constrained Index (USD Hedged) less 15%.While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate or currency exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, and swaps.MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.MFS may invest the fund's assets in other mutual funds advised by MFS that invest in particular investment types rather than invest directly in such investments.For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes. Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The principal risks of investing in the fund are: As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results. Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market. Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest. Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Debt instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could negatively affect the market value and liquidity of a debt instrument.Below investment grade quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Below investment grade quality debt instruments are regarded as having predominantly speculative characteristics. Below investment grade quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments. Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These factors can make foreign investments, especially those tied economically to emerging markets or countries subject to sanctions or the threat of new or modified sanctions, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Emerging Markets Risk: Investments tied economically to emerging markets, especially frontier markets, can involve additional and greater risks than the risks associated with investments in developed markets. Emerging markets can have less developed markets, greater custody and operational risk, less developed legal, regulatory, and accounting systems, greater government involvement in the economy, greater risk of new or inconsistent government treatment of or restrictions on issuers and instruments, and greater political, social, geopolitical, and economic instability than developed markets. Currency Risk: The value of foreign currencies relative to the U.S. dollar fluctuates in response to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions, and changes in currency exchange rates impact the financial condition of companies or other issuers and may change the value in U.S. dollars of investments denominated in foreign currencies. Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, public health, and other conditions, and the fund's performance will be affected by the conditions in the industries, sectors, countries, and regions to which the fund is exposed. Furthermore, investments in particular industries, sectors, countries, or regions may be more volatile than the broader market as a whole. Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended. Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage. Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses. Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction. Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs. Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would. Performance Information The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606. The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests. The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. mfs.com 1-800-225-2606 Class A Bar Chart. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The total return for the three-month period ended March 31, 2026, was (0.50)%. During the period(s) shown in the bar chart, the highest quarterly return was 8.93% (for the calendar quarter ended June 30, 2020) and the lowest quarterly return was (12.35)% (for the calendar quarter ended March 31, 2020). The total return for the three-month period ended 2026-03-31 -0.0050 highest quarterly return 0.0893 2020-06-30 lowest quarterly return -0.1235 2020-03-31 Performance Table. Average Annual Total Returns (For the Periods Ended December 31, 2025) Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes 0.0343 0.0293 0.0477 0.0644 0.0324 0.0477 0.0851 0.0428 0.0567 0.0744 0.0325 0.0460 0.0797 0.0377 0.0514 0.0823 0.0403 0.0546 0.0849 0.0429 0.0567 0.0862 0.0442 0.0577 0.0364 0.0312 0.0495 Returns After Taxes on Distributions 0.0152 0.0074 0.0272 Returns After Taxes on Distributions and Sale of Fund Shares 0.0211 0.0128 0.0279 Reflects no deduction for fees, expenses, or taxes Reflects no deduction for fees, expenses, or taxes Bloomberg Global Aggregate Index 0.0817 -0.0215 0.0126 ICE BofA Global High Yield - Constrained Index (USD Hedged) 0.0845 0.0409 0.0626 After-tax returns are calculated using the historical highest individual U.S. federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. MFS® High Income Fund Summary of Key Information MHITX MHIBX MHICX MHIIX MHIGX MIHRX MHIHX MHIJX MHIKX Investment Objective The fund’s investment objective is to seek total return with an emphasis on high current income, but also considering capital appreciation. Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 11 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. 100000 Shareholder Fees (fees paid directly from your investment): 0.0425 0 0 0 0 0 0 0 0 0.0100 0.0400 0.0100 0 0 0 0 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0045 0.0025 0.0100 0.0100 0 0.0100 0.0050 0.0025 0 0 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0016 0.0006 0.0086 0.0161 0.0161 0.0061 0.0161 0.0111 0.0086 0.0061 0.0051 -0.0001 -0.0001 -0.0001 -0.0001 -0.0001 -0.0001 -0.0001 -0.0001 -0.0001 0.0085 0.0160 0.0160 0.0060 0.0160 0.0110 0.0085 0.0060 0.0050 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 508 687 881 1440 563 807 1075 1709 163 507 875 1709 263 507 875 1709 163 507 875 1709 61 194 339 761 163 507 875 1910 112 352 611 1351 87 273 476 1060 61 194 339 761 51 163 284 640 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 51% of the average value of its portfolio. 0.51 Principal Investment Strategies MFS (Massachusetts Financial Services Company, the fund’s investment adviser) normally invests at least 80% of the fund’s net assets in high income debt instruments. Debt instruments include corporate debt instruments, foreign government securities, floating rate loans, and other obligations to repay money borrowed. High income debt instruments are debt instruments that are below investment grade quality. Below investment grade quality debt instruments are debt instruments rated BB+ or lower by S&P or Fitch, or Ba1 or lower by Moody’s, or comparably rated by another Nationally Recognized Statistical Rating Organization ("NRSRO"). MFS may invest the fund’s assets in other types of debt instruments. MFS may invest up to 100% of the fund’s assets in below investment grade quality debt instruments.MFS may invest the fund’s assets in foreign securities. MFS normally invests the fund's assets across different industries and sectors, but MFS may invest a significant percentage of the fund's assets in issuers in a single industry or sector.While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, and swaps.MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes. Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The principal risks of investing in the fund are: As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results. Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market. Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest. Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Debt instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could negatively affect the market value and liquidity of a debt instrument.Below investment grade quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Below investment grade quality debt instruments are regarded as having predominantly speculative characteristics. Below investment grade quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments. Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These factors can make foreign investments, especially those tied economically to countries with developing economies or countries subject to sanctions or the threat of new or modified sanctions, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, public health, and other conditions, and the fund's performance will be affected by the conditions in the industries, sectors, countries, and regions to which the fund is exposed. Furthermore, investments in particular industries, sectors, countries, or regions may be more volatile than the broader market as a whole. Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended. Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage. Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses. Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction. Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs. Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would. Performance Information The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606. The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests. The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. mfs.com 1-800-225-2606 Class A Bar Chart. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The total return for the three-month period ended March 31, 2026, was (0.68)%. During the period(s) shown in the bar chart, the highest quarterly return was 7.00% (for the calendar quarter ended March 31, 2019) and the lowest quarterly return was (10.23)% (for the calendar quarter ended March 31, 2020). The total return for the three-month period ended 2026-03-31 -0.0068 highest quarterly return 0.0700 2019-03-31 lowest quarterly return -0.1023 2020-03-31 Performance Table. Average Annual Total Returns (For the Periods Ended December 31, 2025) Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes 0.0353 0.0255 0.0465 0.0688 0.0294 0.0462 0.0896 0.0396 0.0551 0.0753 0.0287 0.0446 0.0808 0.0332 0.0492 0.0870 0.0364 0.0521 0.0895 0.0397 0.0554 0.0907 0.0408 0.0562 0.0407 0.0281 0.0479 Returns After Taxes on Distributions 0.0152 0.0055 0.0258 Returns After Taxes on Distributions and Sale of Fund Shares 0.0235 0.0112 0.0268 Reflects no deduction for fees, expenses, or taxes Reflects no deduction for fees, expenses, or taxes Bloomberg U.S. Aggregate Bond Index 0.0730 -0.0036 0.0201 Bloomberg U.S. Corporate High-Yield 2% Issuer Capped Index 0.0862 0.0450 0.0652 After-tax returns are calculated using the historical highest individual U.S. federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. MFS® High Yield Pooled Portfolio Summary of Key Information HYPPX Investment Objective The fund’s investment objective is to seek total return with an emphasis on high current income, but also considering capital appreciation. Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Shareholder Fees (fees paid directly from your investment): 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0000 0.0003 0.0003 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods; your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 3 10 17 39 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 50% of the average value of its portfolio. 0.50 Principal Investment Strategies MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests at least 80% of the fund’s net assets in high yielding debt instruments. Debt instruments include corporate debt instruments, foreign government securities, floating rate loans, and other obligations to repay money borrowed. High yield debt instruments are debt instruments that are below investment grade quality. Below investment grade quality debt instruments are debt instruments rated BB+ or lower by S&P or Fitch, or Ba1 or lower by Moody’s, or comparably rated by another Nationally Recognized Statistical Rating Organization ("NRSRO"). MFS may invest the fund’s assets in other types of debt instruments. MFS may invest up to 100% of the fund’s assets in below investment grade quality debt instruments.MFS may invest the fund’s assets in foreign securities. MFS normally invests the fund's assets across different industries and sectors, but MFS may invest a significant percentage of the fund's assets in issuers in a single industry or sector.While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, and swaps.MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes. Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The principal risks of investing in the fund are: As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results. Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market. Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest. Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Debt instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could negatively affect the market value and liquidity of a debt instrument.Below investment grade quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Below investment grade quality debt instruments are regarded as having predominantly speculative characteristics. Below investment grade quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments. Foreign Risk: Exposure to foreign markets through issuers or currencies can involve additional risks relating to market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These factors can make foreign investments, especially those tied economically to countries with developing economies or countries subject to sanctions or the threat of new or modified sanctions, more volatile and less liquid than U.S. investments. In addition, foreign markets can react differently to these conditions than the U.S. market. Focus Risk: Issuers in a single industry, sector, country, or region can react similarly to market, currency, political, economic, regulatory, geopolitical, environmental, public health, and other conditions, and the fund's performance will be affected by the conditions in the industries, sectors, countries, and regions to which the fund is exposed. Furthermore, investments in particular industries, sectors, countries, or regions may be more volatile than the broader market as a whole. Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended. Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage. Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses. Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction. Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs. Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would. Performance Information The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606. The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests. The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. mfs.com 1-800-225-2606 Bar Chart. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The total return for the three-month period ended March 31, 2026, was (0.40)%. During the period(s) shown in the bar chart, the highest quarterly return was 7.30% (for the calendar quarter ended December 31, 2023) and the lowest quarterly return was (10.58)% (for the calendar quarter ended March 31, 2020). The total return for the three-month period ended 2026-03-31 -0.0040 highest quarterly return 0.0730 2023-12-31 lowest quarterly return -0.1058 2020-03-31 Performance Table. Average Annual Total Returns (For the Periods Ended December 31, 2025) Returns Before Taxes 0.0944 0.0461 0.0619 Returns After Taxes on Distributions 0.0636 0.0193 0.0353 Returns After Taxes on Distributions and Sale of Fund Shares 0.0551 0.0231 0.0357 Reflects no deduction for fees, expenses, or taxes Reflects no deduction for fees, expenses, or taxes Bloomberg U.S. Aggregate Bond Index 0.0730 -0.0036 0.0201 Bloomberg U.S. Corporate High-Yield 2% Issuer Capped Index 0.0862 0.0450 0.0652 After-tax returns are calculated using the historical highest individual U.S. federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. MFS® Municipal High Income Fund Summary of Key Information MMHYX MMHBX MMHCX MMIIX MMHKX Investment Objective The fund’s investment objective is to seek total return with an emphasis on high income exempt from U.S. federal income tax, but also considering capital appreciation. Fees and Expenses This table describes the fees and expenses that you may pay when you buy, hold, and sell shares of the fund. Investors may also pay commissions or other fees to their financial intermediaries when they buy, hold, and sell shares of the fund, which are not reflected below. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. More information about these and other waivers and reductions is available from your financial intermediary and in “Sales Charges and Waivers and Reductions” on page 11 and “Appendix A – Waivers and Reductions of Sales Charges” on page A-1 of the fund’s prospectus. You may qualify for sales charge reductions if, with respect to Class A shares, you and certain members of your family invest, or agree to invest in the future, at least $100,000 in MFS funds. 100000 Shareholder Fees (fees paid directly from your investment): 0.0425 0 0 0 0 0.0100 0.0400 0.0100 0 0 Annual Fund Operating Expenses (expenses that you pay each year as a percentage of the value of your investment): 0.0053 0.0053 0.0053 0.0053 0.0053 0 0.0100 0.0100 0 0 0.0020 0.0020 0.0020 0.0020 0.0012 0.0073 0.0173 0.0173 0.0073 0.0065 -0.0007 -0.0032 -0.0007 -0.0007 -0.0008 0.0066 0.0141 0.0166 0.0066 0.0057 Example This example is intended to help you compare the cost of investing in the fund with the cost of investing in other mutual funds. The example assumes that: you invest $10,000 in the fund for the time periods indicated and you redeem your shares at the end of the time periods (unless otherwise indicated); your investment has a 5% return each year; and the fund’s operating expenses remain the same. Although your actual costs will likely be higher or lower, under these assumptions your costs would be: 490 642 807 1287 544 814 1109 1748 144 514 909 1748 269 538 932 1770 169 538 932 1770 67 226 399 900 58 200 354 803 Portfolio Turnover The fund pays transaction costs, such as commissions, when it buys and sells securities (or “turns over” its portfolio). A higher portfolio turnover rate may indicate higher transaction costs and may result in higher taxes when shares are held in a taxable account. These transaction costs, which are not reflected in “Annual Fund Operating Expenses” or in the “Example,” affect the fund’s performance. During the most recent fiscal year, the fund’s portfolio turnover rate was 9% of the average value of its portfolio. 0.09 Principal Investment Strategies MFS (Massachusetts Financial Services Company, the fund's investment adviser) normally invests the fund’s assets primarily in municipal instruments. MFS defines municipal instruments as debt instruments issued by or for states, territories, or possessions of the United States or by their political subdivisions, agencies, authorities, or other government entities, to raise money for a variety of public and private purposes, including general financing for state and local governments, or financing for a specific project or public facility. Municipal instruments include general obligation bonds of municipalities, state or local governments, project or revenue-specific bonds, municipal lease obligations, and pre-refunded or escrowed bonds. The fund invests, under normal market conditions, at least 80% of its net assets in securities and other investments, the interest on which is exempt from U.S. federal income tax. However, interest from the fund’s investments may be subject to the U.S. federal alternative minimum tax.MFS may invest 25% or more of the fund’s total assets in municipal instruments that finance similar types of projects, such as those relating to education, healthcare, housing, utilities, water, or sewers. MFS may invest up to 100% of the fund’s assets in below investment grade quality debt instruments.MFS may invest a significant percentage of the fund's assets in issuers in a single state, territory, or possession, or a small number of states, territories, or possessions.While MFS may use derivatives for any investment purpose, to the extent MFS uses derivatives, MFS expects to use derivatives primarily to increase or decrease exposure to a particular market, segment of the market, or security, to increase or decrease interest rate exposure, or as alternatives to direct investments. Derivatives include futures, forward contracts, options, inverse floating rate instruments, and swaps.MFS uses an active bottom-up investment approach to buying and selling investments for the fund. Investments are selected primarily based on fundamental analysis of individual instruments and their issuers. Quantitative screening tools that systematically evaluate instruments may also be considered. In structuring the fund, MFS also considers top-down factors.For purposes of the fund's 80% policy, net assets include the amount of any borrowings for investment purposes. Principal Risks As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency.The principal risks of investing in the fund are: As with any mutual fund, the fund may not achieve its objective and/or you could lose money on your investment in the fund. An investment in the fund is not a bank deposit and is not insured or guaranteed by the Federal Deposit Insurance Corporation or any other governmental agency. Investment Selection Risk: MFS' investment analysis and its selection of investments may not produce the intended results and/or can lead to an investment focus that results in the fund underperforming other funds with similar investment strategies and/or underperforming the markets in which the fund invests. In addition, to the extent MFS considers quantitative tools in managing the fund, such tools may not produce the intended results. Debt Market Risk: Debt markets can be volatile and can decline significantly in response to changes in, or investor perceptions of, issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. These conditions can affect a single instrument, issuer, or borrower, a particular type of instrument, issuer, or borrower, a segment of the debt markets or the debt markets generally. Certain events can have a dramatic adverse effect on debt markets and may lead to periods of high volatility and reduced liquidity in a debt market or segment of a debt market. Interest Rate Risk: In general, the price of a debt instrument falls when interest rates rise and rises when interest rates fall. Interest rate risk is generally greater for instruments with longer maturities or durations, or that do not pay current interest. Credit Risk: The price of a debt instrument depends, in part, on the credit quality of the issuer, borrower, counterparty, or other entity responsible for payment, or underlying collateral or assets and the terms of the instrument. The price of a debt instrument can decline in response to changes in, or perceptions of, the financial condition of the issuer, borrower, counterparty, or other entity, or underlying collateral or assets, or changes in, or perceptions of, specific or general market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions. Debt instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could negatively affect the market value and liquidity of a debt instrument.The credit quality of, and the ability to pay principal and interest when due by, an issuer of a municipal instrument depends on the credit quality of the entity supporting the municipal instrument, how essential any services supported by the municipal instrument are, the sufficiency of any revenues or taxes that support the municipal instrument, and/or the willingness or ability of the appropriate government entity to approve any appropriations necessary to support the municipal instrument. In addition, the price of a municipal instrument also depends on its credit quality and ability to meet the credit support obligations of any insurer or other entity providing credit support to a municipal instrument. Below investment grade quality debt instruments (commonly referred to as “high yield securities” or “junk bonds”) can involve a substantially greater risk of default or can already be in default, and their values can decline significantly. Below investment grade quality debt instruments are regarded as having predominantly speculative characteristics. Below investment grade quality debt instruments tend to be more sensitive to adverse news about the issuer, or the market or economy in general, than higher quality debt instruments. Municipal Risk: The price of a municipal instrument can be volatile and significantly affected by adverse tax changes or court rulings, legislative or political changes, changes in specific or general market and economic conditions and developments, and the financial condition of municipal issuers and insurers. Because many municipal instruments are issued to finance similar projects, conditions in certain industries can significantly affect the fund and the overall municipal market. Municipal instruments may be more susceptible to downgrades or defaults during economic downturns or similar periods of economic stress, which in turn could affect the market values and marketability of many or all municipal obligations of issuers in a state, U.S. territory, or possession. In addition, because some municipal obligations may be secured or guaranteed by banks and other institutions, the risk associated with investments in such municipal securities could increase if the banking or financial sector suffers an economic downturn and/or if the credit ratings of the institutions issuing the guarantee are downgraded or at risk of being downgraded by a national rating organization. If such events occur, the value of the security could decrease or the value could be lost entirely, and it may be difficult or impossible to sell the security at the time and the price that normally prevails in the market. Focus Risk: The fund’s performance will be closely tied to the issuer, market, economic, industry, political, regulatory, geopolitical, environmental, public health, and other conditions in the states, territories, and possessions of the United States in which the fund's assets are invested. If MFS invests a significant percentage of the fund's assets in a single state, territory, or possession, or a small number of states, territories, or possessions, these conditions will have a significant impact on the fund's performance and the fund's performance may be more volatile than the performance of more geographically-diversified funds. Prepayment/Extension Risk: Instruments subject to prepayment and/or extension can reduce the potential for gain for the instrument’s holders if the instrument is prepaid and increase the potential for loss if the maturity of the instrument is extended. Derivatives Risk: Derivatives can be highly volatile and involve risks in addition to the risks of the underlying indicator(s) on which the derivative is based. Gains or losses from derivatives can be substantially greater than the derivatives’ original cost. Derivatives can involve leverage. Leveraging Risk: Leverage involves investment exposure in an amount exceeding the initial investment. Leverage can cause increased volatility by magnifying gains or losses. Counterparty and Third Party Risk: Transactions involving a counterparty or third party other than the issuer of the instrument are subject to the credit risk of the counterparty or third party, and to the counterparty’s or third party’s ability or willingness to perform in accordance with the terms of the transaction. Liquidity Risk: It may be difficult to value, and it may not be possible to sell, certain investments, types of investments, and/or investments in certain segments of the market, and the fund may have to sell certain of these investments at prices or times that are not advantageous in order to meet redemptions or other cash needs. Large Shareholder Risk: From time to time, shareholders of the fund (which may include institutional investors, financial intermediaries, or other MFS funds) may make relatively large redemptions or purchases of fund shares. These transactions may cause the fund to sell securities or invest additional cash, as the case may be, at disadvantageous prices. Redemptions of a large number of shares also may increase transaction and other costs or have adverse tax consequences for shareholders of the fund by requiring a sale of portfolio securities. Purchases of a large number of shares may adversely affect the fund's performance to the extent that it takes time to invest new cash and the fund maintains a larger cash position than it ordinarily would. Performance Information The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests.The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. Updated performance is available online at mfs.com or by calling 1-800-225-2606. The bar chart and performance table below are intended to provide some indication of the risks of investing in the fund by showing changes in the fund’s performance over time and how the fund’s performance over time compares with that of (i) a broad measure of market performance and (ii) the performance of an additional index that the adviser believes more closely reflects the market segments in which the fund invests. The fund’s past performance (before and after taxes) does not necessarily indicate how the fund will perform in the future. mfs.com 1-800-225-2606 Class A Bar Chart. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The bar chart does not take into account any sales charges (loads) that you may be required to pay upon purchase or redemption of the fund’s shares. If these sales charges were included, they would reduce the returns shown. The total return for the three-month period ended March 31, 2026, was 0.27%. During the period(s) shown in the bar chart, the highest quarterly return was 8.26% (for the calendar quarter ended December 31, 2023) and the lowest quarterly return was (7.11)% (for the calendar quarter ended March 31, 2022). The total return for the three-month period ended 2026-03-31 0.0027 highest quarterly return 0.0826 2023-12-31 lowest quarterly return -0.0711 2022-03-31 Performance Table. Average Annual Total Returns (For the Periods Ended December 31, 2025) Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes Returns Before Taxes -0.0121 0.0022 0.0245 0.0150 0.0031 0.0225 0.0350 0.0131 0.0308 0.0358 0.0139 0.0313 -0.0089 0.0044 0.0262 Returns After Taxes on Distributions -0.0092 0.0040 0.0258 Returns After Taxes on Distributions and Sale of Fund Shares 0.0116 0.0116 0.0287 Reflects no deduction for fees, expenses, or taxes Reflects no deduction for fees, expenses, or taxes Bloomberg Municipal Bond Index 0.0425 0.0080 0.0234 MFS Municipal High Income Fund Blended Index 0.0346 0.0156 0.0332 As of December 31, 2025, the MFS Municipal High Income Fund Blended Index (the Blended Index) consisted of the following indices and weightings: 50% Bloomberg Municipal Bond Index and 50% Bloomberg Municipal High Yield 2% Issuer Capped Index. The components and weightings of the Blended Index may have differed during the periods and may differ in the future.After-tax returns are calculated using the historical highest individual U.S. federal marginal income tax rates and do not reflect the impact of state and local taxes. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. As of December 31, 2025, the MFS Municipal High Income Fund Blended Index (the Blended Index) consisted of the following indices and weightings: 50% Bloomberg Municipal Bond Index and 50% Bloomberg Municipal High Yield 2% Issuer Capped Index. The components and weightings of the Blended Index may have differed during the periods and may differ in the future. Your actual after-tax returns will depend on your own tax situation, and may differ from those shown. The after-tax returns shown are not relevant to investors who hold their shares through tax-advantaged arrangements, such as 401(k) plans or individual retirement accounts (IRAs). The after-tax returns are shown for only one of the fund’s classes of shares, and after-tax returns for the fund’s other classes of shares will vary from the returns shown. 485BPOS 2026-05-29 2026-01-31 0000225604 2026-05-28 false ~ http://xbrl.sec.gov/rr/role/ShareholderFeesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002428Summary1Member row dei_LegalEntityAxis compact cik0000225604_S000002428Member ~ ~ http://xbrl.sec.gov/rr/role/OperatingExpensesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002428Summary1Member row dei_LegalEntityAxis compact cik0000225604_S000002428Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExample row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002428Summary1Member row dei_LegalEntityAxis compact cik0000225604_S000002428Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExampleNoRedemption row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002428Summary1Member row dei_LegalEntityAxis compact cik0000225604_S000002428Member ~ ~ http://xbrl.sec.gov/rr/role/PerformanceTableData column period compact * row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002428Summary1Member row dei_LegalEntityAxis compact cik0000225604_S000002428Member row oef_ClassAxis compact * row oef_PerformanceMeasureAxis compact * ~ ~ http://xbrl.sec.gov/rr/role/ShareholderFeesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002427Summary2Member row dei_LegalEntityAxis compact cik0000225604_S000002427Member ~ ~ http://xbrl.sec.gov/rr/role/OperatingExpensesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002427Summary2Member row dei_LegalEntityAxis compact cik0000225604_S000002427Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExample row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002427Summary2Member row dei_LegalEntityAxis compact cik0000225604_S000002427Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExampleNoRedemption row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002427Summary2Member row dei_LegalEntityAxis compact cik0000225604_S000002427Member ~ ~ http://xbrl.sec.gov/rr/role/PerformanceTableData column period compact * row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002427Summary2Member row dei_LegalEntityAxis compact cik0000225604_S000002427Member row oef_ClassAxis compact * row oef_PerformanceMeasureAxis compact * ~ ~ http://xbrl.sec.gov/rr/role/ShareholderFeesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000038262Summary3Member row dei_LegalEntityAxis compact cik0000225604_S000038262Member ~ ~ http://xbrl.sec.gov/rr/role/OperatingExpensesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000038262Summary3Member row dei_LegalEntityAxis compact cik0000225604_S000038262Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExample row dei_DocumentInformationDocumentAxis compact cik0000225604_S000038262Summary3Member row dei_LegalEntityAxis compact cik0000225604_S000038262Member ~ ~ http://xbrl.sec.gov/rr/role/PerformanceTableData column period compact * row dei_DocumentInformationDocumentAxis compact cik0000225604_S000038262Summary3Member row dei_LegalEntityAxis compact cik0000225604_S000038262Member row oef_ClassAxis compact * row oef_PerformanceMeasureAxis compact * ~ ~ http://xbrl.sec.gov/rr/role/ShareholderFeesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002429Summary4Member row dei_LegalEntityAxis compact cik0000225604_S000002429Member ~ ~ http://xbrl.sec.gov/rr/role/OperatingExpensesData row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002429Summary4Member row dei_LegalEntityAxis compact cik0000225604_S000002429Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExample row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002429Summary4Member row dei_LegalEntityAxis compact cik0000225604_S000002429Member ~ ~ http://xbrl.sec.gov/rr/role/ExpenseExampleNoRedemption row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002429Summary4Member row dei_LegalEntityAxis compact cik0000225604_S000002429Member ~ ~ http://xbrl.sec.gov/rr/role/PerformanceTableData column period compact * row dei_DocumentInformationDocumentAxis compact cik0000225604_S000002429Summary4Member row dei_LegalEntityAxis compact cik0000225604_S000002429Member row oef_ClassAxis compact * row oef_PerformanceMeasureAxis compact * ~ 0.1292 0.071 -0.031 0.1399 0.0404 0.0271 -0.1003 0.1242 0.0831 0.0824 0.1338 0.0613 -0.033 0.1443 0.0442 0.032 -0.1056 0.1218 0.0658 0.0869 0.1449 0.073 -0.0225 0.1544 0.0501 0.0416 -0.0986 0.1334 0.0756 0.0944 0.0181 0.0764 0.0208 0.096 0.0338 0.053 -0.1384 0.0614 0.0709 0.0351 This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 May 31, 2027 ~ http://www.MFS.com/20260131/role/RRRiskTable3 ~ ~ http://www.MFS.com/20260131/role/RRRiskTable5 ~ ~ http://www.MFS.com/20260131/role/RRRiskTable7 ~ ~ http://www.MFS.com/20260131/role/RRRiskTable9 ~ This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. Massachusetts Financial Services Company (MFS) has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least May 31, 2027. MFS has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, certain tax reclaim recovery expenses (including contingency fees and closing agreement expenses), and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity, and fees and expenses associated with investments in investment companies and other similar investment vehicles), such that "Total Annual Fund Operating Expenses" do not exceed 1.00% of the class' average daily net assets annually for each of Class A and Class R3 shares, 1.75% of the class' average daily net assets annually for each of Class B, Class C, and Class R1 shares, 0.75% of the class' average daily net assets annually for each of Class I and Class R4 shares, 1.25% of the class' average daily net assets annually for Class R2 shares, and 0.67% of the class' average daily net assets annually for Class R6 shares. This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least May 31, 2027. Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years. This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. Massachusetts Financial Services Company (MFS) has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least May 31, 2027. Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years. This contingent deferred sales charge (CDSC) applies to shares purchased without an initial sales charge and redeemed within 18 months of purchase. Massachusetts Financial Services Company (MFS) has agreed in writing to waive at least 0.01% of the fund's management fee as part of an agreement pursuant to which MFS has agreed to reduce its management fee by a specified amount if certain MFS mutual fund assets exceed thresholds agreed to by MFS and the fund's Board of Trustees. The agreement to waive at least 0.01% of the management fee will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least May 31, 2027. MFS Fund Distributors, Inc. (MFD), has agreed in writing to waive the Class B service fee to 0.00% of the class' average daily net assets annually until modified by the fund’s Board of Trustees, but such agreement will continue until at least May 31, 2027. MFS has agreed in writing to bear the fund's expenses, excluding interest, taxes, extraordinary expenses, brokerage and transaction costs, certain tax reclaim recovery expenses (including contingency fees and closing agreement expenses), and investment-related expenses (such as interest and borrowing expenses incurred in connection with the fund's investment activity), such that "Total Annual Fund Operating Expenses" do not exceed 0.59% of the class' average daily net assets annually for each of Class A and Class I shares, 1.34% of the class' average daily net assets annually for Class B shares, 1.59% of the class' average daily net assets annually for Class C shares, and 0.50% of the class' average daily net assets annually for Class R6 shares. ("Other Expenses" include 0.07% of interest and/or investment-related expenses incurred in connection with the fund's investment activity which are excluded from the expense limitation described in the prior sentence.) This written agreement will continue until modified by the fund's Board of Trustees, but such agreement will continue until at least May 31, 2027. Shares automatically convert to Class A shares approximately eight years after purchase; therefore, the expense examples reflect Class A share expenses after eight years.