Thirty-four persons became ill with acute fluoride poisoning shortly after drinking water in an elementary school in Los Lunas, NM. The water supply of the school was supplemented with a concentrated sodium fluoride solution designed to raise fluoride levels in drinking water to 1 to 5 ppm. Two water samples taken after the incident was reported showed elevated fluoride levels of 375 and 93.5 ppm. Malfunction of a relay switch controlling dilution of the fluoride solution produced toxic fluoride levels in the water. The symptoms of all patients were mild and generally of short duration, with two thirds of patients reporting complete resolution within 24 hours. The bitter, salty taste of the water deterred consumption of large amounts and prevented serious intoxication.