In early September 2006, the Paris Departmental Health and Social Affairs Directorate (DASS) detected a cluster of Legionnaires' disease cases in a southern district of the capital. The investigation launched by the Île-de-France Regional Health and Social Affairs Committee (CIRE) involved various regional DASS offices, the Departmental Health and Social Affairs Service (DESUS), the Institute for Epidemiology, Public Health and Prevention (STIIIC), the National Institute of Public Health (INVS), and the National Reference Center for Legionella in Lyon. It comprised three components: epidemiological, microbiological, and environmental. Patients were questioned about their travel history in the 10 days preceding the onset of symptoms, and cases were sought nationwide as well as through Ewgli and EWRS. In total, 29 cases were identified, including 2 cases via Ewgli. All patients presented with pneumonia and a positive urinary antigen test (Lp1). A clinical strain was isolated from 7 patients. The onset of symptoms ranged from July 28 to September 3, 2006. The median age of the patients was 53.5 years [min=21; max=86], with a sex ratio of 6.2 and a case-fatality rate of 10% (3 deaths). The patients interviewed all reported having visited an area centered around the Austerlitz train station and the Bibliothèque nationale de France. In this area, 25 sites totaling 95 cooling towers were investigated. Among these sites, 5 had shown abnormal results during regulatory inspections with no evidence of a return to normal. All contaminated cooling towers underwent corrective measures supervised by the Stiiic. From a bacteriological perspective, the 7 clinical strains from patients were compared by the CNR to environmental strains from three sites using pulsed-field gel electrophoresis. The strains from 6 patients were found to be identical to those from the cooling towers at Austerlitz Station, identified for the first time by the CNR. The strain from the 7th patient was found to be different from the strains isolated in the other TARs and also different from the endemic "Paris" strain. The TARs at the station were suspended as of September 7 and permanently discontinued on September 9, 2006. No new cases were recorded after September 3. (R.A.)