Following the heat wave of August 2003, several studies were launched at the InVS to investigate the factors contributing to the mortality observed during that period. This report presents the results of two case-control studies conducted in nursing homes for the elderly (EHPAD). The first study covers 350 facilities (175 cases and 175 controls) in regions where both mortality and temperatures were highest during the summer of 2003. [...] The second study covers 628 subjects (314 who died between August 5 and 15, 2003, and 314 who were alive as of September 1, 2003, all residents of an EHPAD that experienced high mortality during the summer of 2003). The objective of this study was to identify individual risk factors for mortality observed between August 5 and 15, 2003. [...] The results of these two studies highlight, in addition to the subject’s health status, the predominant role of behavioral adaptation to heat and the physical conditions that make such adaptation possible. Information and organizational measures would have had a significant impact on survival rates if they had been implemented preventively. (R.A.)