Leptospirosis, an acute infectious disease caused by a spirochete of the genus Leptospira following a 10-day incubation period, has not been a reportable disease since 1986. Current surveillance is based on data from the National Reference Center (CNR) for Leptospira, which receives requests for serology from hospital laboratories or biological and medical testing facilities throughout the country. On June 14, 2001, the Charente-Maritime Departmental Health and Social Affairs Directorate (DDASS) was informed by the Rochefort subprefecture of the onset of clinical signs suggestive of leptospirosis in five adolescents hospitalized in Rochefort. The patients had been swimming with about thirty other adolescents in a canal. On June 26, the Ddass and the InVS, in collaboration with the Interregional Epidemiology Unit (CIRE), decided to dispatch a team to the field to conduct an epidemiological investigation. This article presents the materials, methods, and results of that investigation. The on-site investigation not only made it possible to describe the cluster of cases but also to raise awareness among physicians and private laboratories regarding the existence of the disease and the reporting of suspected cases, to reignite the discussion on the role of the CNR, the alert mechanism, and the possibility of antibiotic prophylaxis for exposed individuals, and provided guidance for drafting an investigation guide. (adapted from the text)