This article describes the epidemiological investigation conducted following the reporting of two cases of Legionnaires' disease among residents of Harnes. The discovery of significant contamination in the cooling systems of a company located less than one kilometer from their homes prompted the DDASS to immediately search for additional cases among general practitioners in the area and at the Lens Hospital Center (CH). By December 2, a total of 9 cases of Legionnaires' disease had been identified. All patients resided in Harnes and neighboring municipalities, and their symptoms had begun in November 2003. An epidemiological investigation was immediately launched by the DDASS and the Northern Interregional Epidemiology Unit (CIRE) to confirm the epidemic nature of the outbreak, assess its scope, identify the origin and source of the contamination, and implement appropriate control measures.