The Supreme Court ruling on the Affordable Care Act upheld the individual mandate and made the expansion of Medicaid coverage to nonelderly adults with incomes below 138 percent of the federal poverty level optional for states. The law intended to require all states to undertake the Medicaid expansion. In this brief, Urban Institute researchers estimate the number of uninsured Americans in each state who would be eligible for Medicaid if every state takes the option of expanding Medicaid coverage. The authors also estimate the number of uninsured Americans who are at risk of not being covered because the Medicaid expansion is now optional.