We observed the daily trend in mortality rates during the 2003 heat wave in 13 of France's largest cities. Mortality data were collected from July 25 to September 15 each year from 1999 through 2003. The conjunction of a maximum temperature of 35 degrees C and a minimum temperature of 20 degrees C was exceptional in 7 cities. An excess mortality rate was observed in the 13 towns, with disparities from +4% (Lille) to +142% (Paris).