Deaths of patients in state psychiatric hospitals in the New York City area during heat waves and control periods from 1950 to 1984 were compared with those in the overall New York City population during the same periods. Psychiatric patients had twice the risk of dying during a heat wave than the general population. The risk for patients was higher in the 1950s, before the introduction of antipsychotic medications, than in the 1980s, but was highest in the 1970s, when higher dosages of antipsychotic medication were commonly used. During 1980 to 1984 the risk was the same for both populations, suggesting that both psychiatric illness and antipsychotic medications increase patients' risk of dying during heat waves but that risk can be eliminated with preventive measures.