The US Environmental Protection Agency recently set a new maximum contaminant level (MCL) for arsenic in drinking water of 10 micro g/l. In this paper, we review the completeness and accuracy of drinking water arsenic occurrence data in the United States and identify populations exposed to elevated arsenic concentrations that would be suitable for epidemiological studies of arsenic health effects. Using existing data from the Environmental Protection Agency Arsenic Occurrence and Exposure Database and additional data from state health and environment departments and water utilities, we identified 33 counties in 11 states with an estimated mean drinking water arsenic concentration of 10 micro g/l or greater. A total of 11 of these 'confirmed' counties had an estimated mean arsenic concentration of 20 micro g/l or more and two had an estimated mean arsenic concentration 50 micro g/l or more. Based on census data, between 1950 and 1999 there were approximately 51.1 million person-years of exposure to drinking water arsenic at levels of 10 micro g/l or more, 8.2 million at levels of 20 micro g/l or more arsenic and 0.9 million at levels of 50 micro g/l or more. Mortality and incidence of diseases known to be associated with arsenic exposure can and should be examined in these counties as part of a comprehensive assessment of arsenic health effects in US populations.