During a heat wave in July 1969, the authors encountered three fatal cases of heat stroke. All the patients had been treated with phenothiazines, which affect temperature regulation and suppress sweating, and two with anti-Parkinsonian agents, which also suppress sweating. The authors stress the need for lower phenothiazine dosages where possible, limited use of supplemental anticholinergic agents, and awareness of the susceptibility of phenothiazine-treated patients to the hazards of extreme temperatures.Abstract Teaser