Between November 5, 2003, and January 22, 2004, 86 cases of Legionnaires' disease were reported in the Pas-de-Calais department. This Legionnaires' disease outbreak is the largest ever recorded in France in terms of duration, geographic scope, and number of cases involved. To complement the epidemiological, microbiological, and environmental investigations conducted, a case-control study was initiated to identify factors associated with the onset of the disease: factors related to the host, their living environment, and their lifestyle habits.