The National Death Index was used to monitor a cohort of 72,740 persons for whom information on household air conditioning was available, between April 1980 and December 1985. 2275 deaths occurred. The study attempted to establish whether people in households with air conditioning experienced lower death rates during hot weather. Both central and room air conditioning were considered. Statistically significant benefits were observed for central air conditioning versus no air conditioning for the overall total, for females, for persons not in the labour force and for those living in fewer than six rooms. These groups had more exposure to air conditioning. The adds ratio for room air conditioning for the whole group was 0.96, which suggested that no real benefit was derived. Suggests some reasons for this lack of demonstrable benefit.