The primary health insurance coverage reforms of the Affordable Care Act took effect on January 1, 2014. Between 2013 and 2016, the uninsurance rate for nonelderly Americans from birth to age 64 fell every year, and 18.5 million more Americans had health insurance coverage in 2016 than in 2013. However, these coverage gains stalled in 2017. Using the American Community Survey, we find that uninsurance increased by 0.2 percentage points between 2016 and 2017 despite a strong economy, meaning 700,000 more uninsured Americans. Increases in uninsurance were concentrated in states that did not expand Medicaid eligibility under the Affordable Health Care Act.