This report examines the effects of the Affordable Care Act’s (ACA) Medicaid expansion on insurance coverage for childless adult citizens with incomes below the federal poverty level. Using data from the National Health Interview Survey, we estimate the effect of the expansion on poor childless adult citizens overall and by age, gender, race, income, education, and self-reported health status. We find that the Medicaid expansion reduced the uninsured rate for this group by 21.4 percentage points, or 47.1 percent, between 2013 and 2015, with large coverage gains across all subgroups. These findings also demonstrate the significant missed opportunities in states that did not expand Medicaid under the ACA.