An acute massive epidemic of arsenic poisoning in Argentina involved 718 subjects. Urine samples were obtained from 307. The 49 with urine arsenic 76-500 micrograms/dl and 12 with urine arsenic greater than 500 micrograms/dL received dimercaprol treatment. Symptomatology increased with the urine arsenic with increased diarrhea, vomiting and systemic symptoms at urine arsenic greater than 75 micrograms/dL.