On February 22, 2002, the National Legionella Reference Center (CNR) reported to the French Institute for Public Health Surveillance (InVS) that the laboratory at the Nice Hospital Center had identified, through serological testing, five cases of Legionnaires’ disease caused by Legionella pneumophila serogroup 6. Three people had died. The Departmental Directorate of Health and Social Affairs (Ddass) of the Alpes-Maritimes was aware of only one case through a mandatory report. An epidemiological investigation was then initiated by the Ddass and the Provence-Alpes-Côte-d'Azur Interregional Epidemiology Unit (CIRE) in collaboration with local partners, involving the formation of a working group comprising clinicians, hospital biologists, epidemiologists, and environmentalists. (R.A.)