A nationwide survey of heavy-metal exposure in children living near primary nonferrous metal smelters demonstrated high urine arsenic levels in children living near a copper smelter in Ajo, Arizona. Airborne smelter emissions and drinking water were the apparent sources of exposure. To determine whether increased arsenic absorption had produced adverse health effects, we conducted an evaluation of 132 Ajo children 5 to 18 years old and compared results with those of 47 children from a comparison town with low arsenic exposure. Environmental testing showed that Ajo's municipal water supply contained arsenic in concentrations of 0.09 mg/l (the EPA standard is 0.05 mg/l); arsenic concentrations in dust averaged 342.2 microgram/g. Urine arsenic levels in Ajo children correlated positively with amount of tap-water consumed (r = .32, p less than. 0002) and with distance of residence from the smelter (r = .20, p less than .02). Tap-water drinkers had significantly higher urine arsenic levels than bottled water drinkers (t = 4.21 p less than .001). Mean urine arsenic levels were significantly higher for children in Ajo (4.75 microgram/100 ml) than for children in the comparison town (1.17 microgram/100 ml). Hair arsenic levels correlated poorly with arsenic exposure. Despite the study population's chronic exposure to elevated environmental levels of arsenic, no clinical or hematologic abnormalities attributable to arsenic were found.