A two-year study was carried out on human subjects of various ages and backgrounds who have been drinking water containing 0.05 mg/l (0.05 ppm) or more arsenic for a period of at least 5 years. A control group that had been exposed to arsenic in drinking water at concentrations less than 0.05 mg/l was used for some aberrations and sister chromatid exchanges in the lymphocytes of the two populations with the amount of arsenic in the water they have been drinking. Besides, a questionnaire was designed to explore the incidence of skin cancer, fetal wastage, any genetic development abnormalities in the family and related parameters. Several other variables, e.g., coffee, wine and cigarette consumption, sex, residence (rural vs. urban) and exposure to chemicals, smelter and pesticides were taken into consideration. The study involved the city of and areas in the vicinity of Reno, Nevada, USA. This study on chromosome aberrations (104 exposed and 86 control individuals) and on sister chromatid exchanges (98 exposed and 83 control individuals) did not show that arsenic at a mean level of 0.109 mg/l has any effect on these parameters. Similarly, no effect of arsenic at these modest concentrations was found on other health parameters studied.