Many Medicaid expansion enrollees with incomes less than 138% of the federal poverty level (FPL) face greater cost-sharing and higher out-of-pocket costs when they tum 65 years old and transition to Medicare. In Medicare, there is a subsidy cliff at 100% FPL, above which beneficiaries are not eligible for comprehensive subsidies that cover medical cost-sharing in Medicare Parts A (inpatient) and B (outpatient). Beneficiaries with incomes between 100-150% FPL and limited assets can qualify for partial financial assistance through a complex set of programs. Take-up of these subsidies is limited, and evidence suggests that Black and Hispanic beneficiaries are more likely to be eligible for but less likely to be aware of subsidy programs vs. White beneficiaries. Moreover, higher cost-sharing has been associated with reductions in necessary care and worse outcomes, and in some cases, widening of racial/ethnic disparities in care. Sixteen states have expanded income and asset eligibility for comprehensive cost-sharing subsidies above federal minimums, but there is little evidence on whether such policies mitigate racial/ethnic disparities. In Aims 1 and 2, we will use linked, individual-level Medicaid and Medicare enrollment, claims, and encounter data from 2016-2023 to follow beneficiaries as they transition from Medicaid to Medicare at age 65 and examine whether the transition between programs leads to disparities in the receipt of Medicare subsidies and contributes to disparities in use, clinical event outcomes, and spending. For Aim 1, we will examine if there are racial/ethnic disparities in Medicare coverage among Medicaid expansion beneficiaries that transition to Medicare-only and use machine learning approaches to identify factors (e.g., policy, health care system, individual) that predict receipt of subsidies among minority vs. White beneficiaries. For Aim 2, we will assess if increases in cost-sharing upon entry to Medicare contribute to racial/ethnic disparities in service use (e.g., outpatient visits), clinical event outcomes (e.g., emergency department visits, hospitalizations, mortality), and spending (out-of-pocket and total costs). We will use a difference-in-difference approach to compare changes in outcomes for beneficiaries just above vs. below income limits for comprehensive cost-sharing subsidies, after vs. before entering Medicare. For Aim 3, we focus on a subset of states that expanded income or asset eligibility for Medicare subsidies above federal minimums in 2008 or later to assess if these policy changes reduce racial/ethnic disparities associated with Medicare cost-sharing. We will use 2006-2023 Medicare data and a difference-indifference approach to examine changes in outcomes for beneficiaries who gain comprehensive cost-sharing subsidies before vs. after state subsidy expansions. In all aims, we will examine whether effects differ for Black and Hispanic vs. White beneficiaries. Findings from this study will inform covera...