The August 2003 heat wave in France was exceptional, both in terms of maximum and minimum temperatures and the associated short-term excess mortality. To anticipate the occurrence of a large-scale epidemic linked to a heat wave, the French Institute for Public Health Surveillance has developed, in collaboration with Météo France, a biometeorological warning system. Its purpose is to alert health authorities three days in advance and to enable the implementation of measures outlined in the National Heat Wave Plan (PNC) from June 1 to September 30. Various meteorological indicators were tested in fourteen pilot cities to determine the most appropriate indicator and thresholds, after which the system was extended to the entire metropolitan area. (R.A.)