Between August 23 and 31, 2004, four cases of Legionnaires' disease caused by Legionella pneumophila serogroup 1 were reported to the Gironde Departmental Health and Social Affairs Directorate (DDASS) among residents of the town of Soulac-sur-Mer (2,819 inhabitants in winter, according to INSEE; approximately 30,000 residents in summer). The article describes the epidemiological and environmental investigation, initiated by the Ddass, the Aquitaine Interregional Epidemiology Unit, and the InVS, to identify a common source of contamination and propose control measures.