Among a population using water from individual wells with elevated levels of arsenic, we have studied hair arsenic concentration and obtained histories of medical conditions. A statistically significant association of water arsenic and hair arsenic is demonstrated. Arsenic ingestion does not seem to be associated with any medical condition in this population. We conclude that above 0.05 (± 0.03) mg 1 −1 of arsenic in drinking water, there is evidence of increased storage of the material in the body as reflected in increasing levels of arsenic in hair.