An outbreak of subacute poisoning occurred among nine members of a family; eight were ill with gastrointestinal symptoms, four developed encephalopathy, and two died. Abnormal liver function tests and leukopenia were common laboratory findings. Epidemiologic and environmental investigations traced the source of arsenic exposure to a farm well with water containing 108 ppm arsenic. The soil adjacent to the well was also contaminated with arsenic, possibly from waste pesticide. Presumably, arsenic gained access to the well through obvious leaks in the well's casing. To our knowledge, this is only the second reported outbreak of fatal arsenic poisoning from contaminated drinking water and one of few instances where illness followed exposure to a toxic substance which was disposed of, or possibly disposed of, in an indiscriminate manner.