In August 2012, seven cases of Legionnaires' disease were reported to the Ile-de-France Regional Health Agency (ARS), including five residents of a nursing home (Ehpad) in Meudon and two community cases living nearby. An active case-finding investigation was launched. The environmental investigation explored all potential sources of contamination, particularly wet cooling towers (TarH). The clinical and environmental Legionella strains isolated were typed at the National Reference Center (CNR). In total, eight cases of Legionnaires’ disease caused by Legionella pneumophila serogroup 1 were recorded, including 5 residents of the nursing home (3 of whom died) and 3 community cases. The reported onset dates ranged from August 1 to August 20, 2012. The average age of the cases was 84 years. The only common factor identified among all cases was exposure to a geographic area with a 2.5-km radius centered on the nursing home. An unannounced inspection conducted on August 20, 2012, revealed contamination of 7,000,000 CFU/L of Legionella pneumophila serogroup 1 in a water treatment plant located in Meudon. A biocidal shock treatment was routinely performed 48 hours prior to environmental surveillance sampling. The three clinical isolates and the environmental isolates from the Meudon long-term care facility were identical. Investigations indicate that the most likely source of contamination was this TarH in Meudon, at least for some of the 8 cases. Given the evolving regulatory framework for TarH surveillance, it would be advisable to maintain a minimum monthly frequency of testing. This episode also highlights the potential lack of representativeness of environmental surveillance samples in cases of prior systematic disinfection. (R.A.)