In France, in 2002, 1,021 cases of Legionnaires' disease were reported to health authorities, representing an incidence of 1.7 cases per 100,000 inhabitants. The sex ratio was 2.9 men to 1 woman. For 53% of patients, the onset of symptoms occurred during the summer months between June 1 and September 30. For the 835 cases for which the disease course was known, the case fatality rate was 13%. On August 27, 2003, the hygiene department of Microbiology Laboratory A (Bacteriology) at the Poitiers University Hospital (CHU) reported two cases of Legionnaires’ disease to the physician at the Departmental Directorate of Health and Social Affairs (DDASS). On the afternoon of August 28, three additional cases were reported. These five individuals lived or worked in Poitiers. Faced with this suspected outbreak, and in order to identify a common source of contamination and implement appropriate control measures, the DDASS immediately: - alerted the Department of Infectious Diseases at the French Institute for Public Health Surveillance (InVS) and the Center-West Interregional Epidemiology Unit (CIRE); - launched a preliminary environmental investigation to identify a geographical area frequented by the first cases; - requested collaboration from the Municipal Health and Hygiene Service (SCHS) of the city of Poitiers and the Regional Directorate for Industry, Research, and the Environment (Drire) to conduct a survey of cooling towers (TAR) in the affected area; - mobilized the relevant departments at the University Hospital: emergency, intensive care, pulmonology, infectious diseases, and the bacteriology laboratory, as well as general practitioners in Poitiers, for the diagnosis and reporting of new cases. Finally, a daily update on the number of people hospitalized for Legionnaires' disease was requested from the University Hospital of Poitiers. (R.A.)