1 We compared the prevalence of signs and symptoms of chronic arsenic poisoning in two rural populations. 2 The arsenic concentration in the drinking water of the exposed population was 0.41 mg/l, and 0.007 mg/l in the control population. 3 The arsenic was present mainly (70%) in its pentavalent form. 4 The objective was to quantitate health effects and risks derived from chronic ingestion of arsenic in contaminated water. 5 In the exposed population, 21.6% of the sample, showed at least one of the cutaneous signs of chronic arsenic poisoning against 2.2% in the control town. 6 Non-specific symptoms were more prevalent in the exposed population and they occurred more frequently in those individuals with skin signs. 7 The relative risk of suffering a particular manifestation of poisoning, ranged from 1.9 to 36 times higher in the exposed population. 8 We estimated the risks above mentioned, which were derived from exposure to minute quantities of arsenic in a known proportion of its oxidation states during a life time period.