The incidence of reported legionellosis in France is estimated based on mandatory reporting (MR) and stood at 2.4 per 100,000 in 2010, with a west-to-east gradient. The objective of this study was to estimate the total number of confirmed cases diagnosed in France, assess the completeness of mandatory reporting, and document diagnostic practices. The study was a retrospective cross-sectional study using the capture-recapture method with two sources: mandatory reporting and hospital laboratories. Case number estimates were calculated using the Chapman method. The survey conducted among laboratories allowed for a description of diagnostic practices. The completeness rate of the DO was estimated at 88.5% [95% CI: 88.0–89.0] and the number of cases at 1,661 [1,621–1,700] in 2010. The adjusted incidence was 2.7 cases per 105 in 2010, and the west-east incidence gradient persisted. All laboratories performed the urine antigen test; 142 (41%) performed culture; and 26 (7%) performed PCR. Of the 158,646 urine tests performed, 0.87% were positive. The percentage of positive tests was higher for the 67 laboratories that routinely concentrated urine (1.06% vs. 0.78%; p<10⁻⁶). This study documents the marked improvement in the completeness of urine culture (88.5% vs. 33% in 1998) and confirms the increased sensitivity of the urine antigen test through urine concentration. Raising awareness among clinicians about this practice would allow for better characterization of the etiology of pneumonia. However, further studies are needed to attempt to explain the observed temporal and spatial variations. (R.A.)