Drinking water maximum contaminant levels (MCL) are established by the U.S. EPA to protect human health. Since 1975, U.S. public water suppliers have reported MCL violations to the national Safe Drinking Water Information System (SDWIS). This study assessed temporal and geographic trends for violations of the 10 mg nitrate-N L-1 MCL in the conterminous U.S. We found that the proportion of systems in violation for nitrate significantly increased from 0.28% to 0.42% of all systems between 1994 and 2009 and then decreased to 0.32% by 2016. The number of people served by systems in violation decreased from 1.5 million in 1997 to 200 000 in 2014. Periodic spikes in people served were often driven by just one large system in violation. On average, Nebraska and Delaware had the greatest proportion of systems in violation (2.7% and 2.4%, respectively), while Ohio and California had the greatest average annual number of people served by systems in violation (278 374 and 139 149 people, respectively). Even though surface water systems that serve more people have been improving over time, groundwater systems in violation and average duration of violations are increasing, indicating persistent nitrate problems in drinking water.