Legionellosis is a disease that has only recently been described, and surveillance for it has been stepped up in recent years. In 2005, 1,527 cases were reported, corresponding to an incidence rate of 2.5 per 100,000 inhabitants. The median age was 61 years [5–100], and the male-to-female ratio was 3.0. The case-fatality rate was 11%. One or more risk factors were identified in 1,084 cases (71%). The majority of cases (91%) were diagnosed using a urinary antigen detection test, and a strain was isolated in 276 cases (18%). The species Legionella pneumophila serogroup 1 accounted for 95% of cases. Risk exposure during the incubation period was reported for 39% of cases. A history of travel was noted for 17% of cases, and 7% had spent part or all of their time in a hospital or clinic. Numerous investigations of clustered cases were conducted. The largest occurred in early May in the northern part of the Lyon metropolitan area, involving 34 cases. Since 1997, the incidence of reported cases of Legionnaires’ disease has been steadily increasing. This likely reflects greater sensitivity and an overall improvement in the surveillance system. In recent years, numerous measures aimed at improving the control and prevention of this disease have been implemented. These efforts must now continue in the field of research to improve understanding of the disease in order to ultimately limit its impact on exposed populations