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, between 2016 and 2017, uninsurance increased by 0.2 percentage points. Using the American Community Survey, we find that uninsurance increased by an additional 0.2 percentage points between 2017 and 2018, meaning 500,000 more uninsured Americans. This increase in uninsurance occurred despite a strong economy and accompanying increases in incomes and employer-sponsored insurance coverage. Recent research suggests that employer-sponsored coverage will decline during the COVID-19 recession, perhaps leaving millions more uninsured.