In mid-July 2001, the Rhône Departmental Directorate of Health and Social Affairs (DDASS) was alerted by the successive reporting of three cases of Legionnaires’ disease among individuals residing in the same district of Lyon (3rd). Initial findings strongly suggested a cluster of cases, similar to those in Paris and Rennes, where the source was external contamination from cooling towers. A descriptive epidemiological investigation was conducted by the DDASS, the City of Lyon’s Urban Ecology Department, and the Municipal Health and Hygiene Service of the City of Villeurbanne, in collaboration with the Rhône-Alpes Interregional Epidemiology Unit (CIRE), the French Institute for Public Health Surveillance (InVS), and the National Legionella Reference Center (CNR). (R.A.)