Since 1987, the surveillance of Legionnaires' disease in France has been based on a mandatory reporting system. Enhanced collaboration with the Legionella Reference Center (CNR) beginning in 1997 has improved the completeness of the reporting system. Since 1997, the number of reported cases of Legionnaires' disease has continued to rise, with an average annual increase of 29%. Improvements in diagnostic methods, a better understanding of the disease, and greater compliance by clinicians with the mandatory reporting system may explain this trend. However, the number of cases (1,021 cases in 2002) remains below 1,124, the number of cases estimated by the completeness study conducted on 1998 data.