Legionellosis is a pneumonia caused by inhalation of Legionella spp. in aerosol water particles. Legionella pneumophila is responsible for approximately 90% of cases; serogroup 1 alone accounts for approximately 85% of cases. Epidemiologic analyses based on pulsed-field gel electrophoresis (PFGE) and sequence-based typing of clinical isolates of L. pneumophila serogroup 1 have detected sporadic, epidemic, and endemic strains. Most cases are sporadic and are associated with strains that have not been identified. A strain is considered endemic to an area when several isolates that have identical PFGE patterns and that cause several epidemiologically unrelated cases of legionellosis are detected in that area. Since 1998, the most prevalent strain endemic to France has been the Paris strain, which was responsible for 12.2% of culture-confirmed cases of legionellosis from 1998 through 2002. The Paris strain has also been detected in clinical samples from several other European countries. (Switzerland, Italy, Spain, and Sweden) and in environmental samples