Label

Element

Value

MFS® Municipal High Income Fund

Risk/Return:

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Supplement to Prospectus [Text Block]

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SUPPLEMENT TO THE PROSPECTUS

The date of this supplement is August 28, 2020.

MFS^® Municipal High Income Fund

Risk/Return [Heading]

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MFS® Municipal High Income Fund

Risk Narrative [Text Block]

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Effective immediately, "Municipal Risk" under the sub-section entitled "Principal Risks" under the main heading entitled "Summary of Key Information" is restated in its entirety as follows:

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. For example, the novel coronavirus (COVID-19) pandemic has significantly stressed the financial resources of many municipal issuers, which may impair a municipal issuer’s ability to meet its financial obligations when due and could adversely impact the value of its bonds, which could negatively impact the performance of the fund.