In 2003, a Heat Health Watch Warning System was developed in France to anticipate heat waves that may result in a large excess of mortality. The system was developed on the basis of a retrospective analysis of mortality and meteorological data in fourteen pilot cities. Several meteorological indicators were tested in relation to levels of excess mortality. Computations of sensibility and specificity were used to choose the meteorological indicators and the cut-offs. An indicator that mixes minimum and maximum temperatures was chosen. The cut-offs were set in order to anticipate events resulting in an excess mortality above 100% in the smallest cities and above 50% in Paris, Lyon, Marseille and Lille. The system was extended nationwide using the 98th percentile of the distribution of minimum and maximum temperatures. A national action plan was set up, using this watch warning system. It was activated on 1st June 2004 on a national scale. The system implies a close cooperation between the French Weather Bureau (Météo France), the National Institute of Health Surveillance (InVS) and the Ministry of Health. The system is supported by a panel of preventive actions, to prevent the sanitary impact of heat waves.