Clin Microbiol InfectAbstract The usefulness of two quantitative real-time PCR assays (qrt-PCRmip targeting Legionella pneumophila, and qrt-PCR16S targeting all Legionella species) performed on lower respiratory tract (LRT) samples for diagnostic and prognostic purposes in 311 patients hospitalized for community-acquired pneumonia (CAP) in Rhône-Alpes (France) was evaluated. The Now Legionella urinary antigen test (UAT) from Binax (Portland, ME, USA) was used as a reference test. Samples were divided into two groups. Group A included 255 CAP patients admitted to Chambery hospital in 2005 and 2006. The Now Legionella UAT was positive in 14 patients. Sensitivities, specificities, positive predictive and negative predictive values for both qrt-PCR tests were 63.6, 98.7, 77.7 and 97.4%, respectively. Group B included 56 consecutive legionellosis patients diagnosed during a 4-year period (2003-2006) at the Grenoble University Hospital. The qrt-PCR16S and qrt-PCRmip displayed a sensitivity of 82.14 and 80.4%, respectively. Among the 70 legionellosis cases, L. pneumophila serogroup 1 was isolated in 15; qrt-PCRmip was positive in another 36, suggesting L. pneumophila infection, whereas the Legionella species involved could not be determined in the remaining 19 cases. The Legionella burden in LRT samples at the time of admission was determined in 46 patients using qrt-PCR16S tests, 44 for qrt-PCR mip groups A and B patients. It varied from 1.9 to 8.35 log(10) DNA copies/mL of LRT sample for qrt-PCR16S and from 1.9 to 8.11 log(10) DNA copies/mL of sample for qrt-PCRmip. High bacterial loads in LRT samples at hospital admission were significantly associated with higher Fine classes, the need for hospitalization in an intensive care unit and for prolonged hospitalization.