In August 2003, several cases of leptospirosis were reported to the Departmental Directorates of Health and Social Affairs, the Interregional Epidemiology Units, and the French Institute for Public Health Surveillance. The number of positive serology results recorded by the Leptospirosis Reference Center was 35% higher than during the same period in 2002. This article presents the investigation conducted to confirm a possible excess of cases. This investigation describes the characteristics of the cases, the search for the source of infection in potential clusters, and the focus of prevention and control measures.