Steep declines in the uninsured population under the Affordable Care Act (ACA) will depend on high enrollment among newly Medicaid-eligible adults. We use the 2009 American Community Survey to model pre-ACA eligibility for comprehensive Medicaid coverage among nonelderly adults. We identify 4.5 million eligible but uninsured adults. We find a Medicaid participation rate of 67% for adults; the rate is 17 percentage points lower than the national Medicaid participation rate for children, and it varies substantially across socioeconomic and demographic subgroups and across states. Achieving substantial increases in coverage under the ACA will require sharp increases in Medicaid participation among adults in some states.