Between November 2003 and January 2004 in the North of France a large outbreak of legionnaire’s disease affected 85 patients. The clinical, biological and radiological characteristics of the patients were investigated to determine factors associated with mortality. Two populations were defined and compared : patients who died within 28 days and those who survived. Eighty-five patients were included in this study. The median age was 75 years. The median fever was 39.3 ± 0.1 degrees. Fifteen patients (17.6%) had at least 3 underlying co-morbidities. Cough, dyspnoea, confusion and diarrhoea were found in respectively 46, 68, 47, and 15% of the patients. The median of urea was 0.7 ± 0.05 g/L, creatinine 16 ± 1.5 mg/L, CRP 332 ± 15 mg/L. On the chest X-ray, lung infiltrates were present in 64% and multilobar in 40%. The overall mortality rate was 21%. In univariate analysis, diabetes mellitus, dyspnoea, urea >0.90g/l and CRP> 350 mg/l were predictive factors of mortality. In multivariate analysis, diabetes mellitus, urea >0.90 g/l, and bilateral infiltrates on chest X ray were retained as independent risk factors for death.