There are large differences in US health insurance coverage by racial and ethnic groups, yet there have been no estimates to date on how implementation of the Affordable Care Act will affect the distribution of coverage by race and ethnicity. We used a microsimulation model to show that racial and ethnic differentials in coverage could be greatly reduced, potentially cutting the eight-percentage-point black-white differential in uninsurance rates by more than half and the nineteen-percentage-point Hispanic-white differential by just under one-quarter. However, blacks and Hispanics are still projected to remain more likely to be uninsured than whites. Achieving low uninsurance under the Affordable Care Act will depend on effective state policies to attain high enrollment in Medicaid and the Children's Health Insurance Program and the new insurance exchanges. Coverage gains among Hispanics will probably depend on adoption of strategies that address language and related barriers to enrollment and retention in California and Texas, where almost half of Hispanics live. If uninsurance is reduced to the extent projected in this analysis, sizable reductions in long-standing racial and ethnic differentials in access to health care and health status are likely to follow.