Patterns of mortality in 11 air-conditioned and 9 un-air-conditioned nursing homes in New York City have been investigated to determine the impacts of heat waves on nursing home occupants and the efficacy of air conditioning in reducing these impacts. During each of four heat waves in 1972 and 1973, the number of deaths in un-air-conditioned nursing homes was significantly greater than the number of deaths expected on the basis of mortality during cooler control periods. In contrast, deaths in air-conditioned nursing homes were not significantly different from expected. The age- and sex-adjusted ratio of relative death rates was 2.29 (95% confidence interval = 1.75-2.98). Approximately 94 excess deaths occurred in the un-air-conditioned nursing homes. On the basis of this research, it is recommended that nursing homes and other institutions for the elderly located in climates like that of New York City be required to provide air conditioning.