In May 2007, eight cases of Legionnaires' disease were reported to the Departmental Directorate of Health and Social Affairs (DDASS) of the Alpes-Maritimes. An active case-finding effort was launched among healthcare professionals in the department, other DDASS offices, and the European EWGLI network. The environmental investigation explored all potential sources of contamination, with a particular focus on cooling towers (TAR). The clinical and environmental Legionella strains isolated were serotyped at the CNR. Nineteen cases of Legionnaires' disease caused by Legionella pneumophila serogroup 1 were recorded. The onset of symptoms ranged from May 15 to June 15, 2007. The average age of the cases was 51 years. The only common factor identified across all cases was exposure to a geographic area located west of Nice. Two of the three isolated clinical strains were identical. The environmental investigation, conducted by the Regional Directorate for Industry, Research, and the Environment (DRIRE) among 60 TAR operators, identified two instances where the action threshold (10⁵ CFU/L) was exceeded and two instances where the alert threshold (10³ CFU/L) was exceeded. None of the isolated environmental strains were identical to the clinical strains. Although the incidence of Legionnaires' disease has been high in the Alpes-Maritimes since 2005, this outbreak is the largest ever reported in the department. The hypothesis of contamination by water treatment systems could not be biologically confirmed, but various epidemiological factors support the hypothesis of contamination by one or more water treatment systems for some of the cases (concentration over time, limited geographic area, and absence of other identified risk factors). A revision of legislation regarding Legionella risk for TARs is recommended: systematic screening for Legionella pneumophila; for any exceedance of the alert threshold, immediate reporting of results to the DRIRE and preservation of strains by laboratories. (R.A.)