Legionella comprises a group of approximately 50 species and 64 serogroups. In France, as in most countries, the species responsible for more than 90% of Legionnaires' disease cases is Legionella pneumophila, and serogroup 1 is found in more than 80% of cases. To determine whether the high proportion of these cases partly reflects an environmental prevalence of these strains, the authors compared the distribution of Legionella species present in French water distribution systems with those responsible for cases of Legionnaires' disease.