The Affordable Care Act (ACA) provides enhanced federal matching funds to states that expand Medicaid to nonelderly adults up to 138% of the federal poverty level (FPL, $17,236/year for an individual in 2019). The ACA enhanced match (93% in 2019, and 90% in 2020 and thereafter) is substantially higher than states’ traditional Medicaid matching rate. A few states have sought Section 1115 demonstration waiver authority from the Centers for Medicare and Medicaid Services (CMS) to receive the substantially higher ACA enhanced match while limiting coverage to individuals at 100% FPL, instead of covering the full 138% FPL ACA group. To date, CMS has allowed states to receive the ACA enhanced Medicaid matching funds only if the entire expansion group is covered. CMS has not approved waiver requests seeking enhanced ACA matching funds for a partial coverage expansion in Arkansas or Massachusetts, while a request is pending in Utah. This brief explores the current rules for partial expansion and explains some of the potential implications for financing and coverage if CMS approves waivers to allow for partial expansion with enhanced matching funds.