Legionellosis has been a notifiable disease since 1987. The surveillance system was strengthened in 1997 with the publication of a circular on the prevention and surveillance of legionellosis, accompanied by a guide for investigating one or more cases of legionellosis. In conjunction with the revision of this investigation guide, the InVS sought to conduct a national survey on the practices of the Ddass in response to an isolated case of Legionnaires' disease. The survey was retrospective. The target population consisted of all DDASS offices in mainland France and overseas territories. The scope of the survey covered isolated cases of Legionnaires' disease (excluding nosocomial and spa-related cases) reported during 2002. This survey yielded 90 responses (91% response rate). Among the responding DDASS offices, 78 (87%) reported receiving 718 notifications of isolated and reported cases of Legionnaires' disease in their department in 2002, representing an average of 9.2 notifications of isolated, non-nosocomial, and non-spa-related Legionnaires' disease. Identification of a source of contamination remains rare given the low number of clinical samples and positive samples relative to regulatory requirements. Indeed, comparisons between clinical and environmental strains are possible in only 5% of home investigations, compared to over 10% in investigations at public-access or medical-social facilities. The share of funding for water samples collected in homes by the DDASS accounts for 66% (188 samples) of all funded samples. Given the results presented and taking into account the limitations on interpretation due to the small sample size, it is important to emphasize the importance of clinical samples, which are an essential prerequisite for any comparison with environmental strains, as well as for identifying a source of contamination and implementing preventive measures upstream (maintenance of water systems, water temperature, etc.). The practices of the DDASS in response to an isolated case of Legionnaires’ disease are highly varied and require, as this survey also calls for, harmonization of practices across the entire country. Furthermore, with the increase in the number of reported cases and the number of investigations conducted, priority should be given to allocating human resources to investigate clusters of cases and community outbreaks. (R.A.)