Despite the fact that the epidemiology of community-acquired pneumonia and nosocomial Legionella infection is well known, there are no specific reports dealing with severe cases of Legionella pneumophila pneumonia admitted to intensive care units. We undertook a prospective study upon 84 patients with a reliable diagnosis of L. pneumophila pneumonia that required ICU admission. The study assessed the prognostic factors, clinical, radiological and outcome variables of both nosocomial (n = 33) and community-acquired (n = 51) cases of L. pneumophila pneumonia. The following variables were more common in nosocomial acquired as compared to community-acquired Legionella pneumonia: Chronic obstructive pulmonary disease (COPD) (64 versus 41%), cardiac disease (39 versus 10%), chronic renal failure (21 versus 4%), alcoholism (54 versus 18%), septic shock (33 versus 16%), and unilateral chest X-ray involvement (61 versus 39%). The crude mortality rate in this study was 30% (25 of 84) with no differences when comparing mortality between nosocomial (9, 27%) to community-acquired (16, 31%) types. The univariate analysis showed that cardiac disease, diabetes mellitus, creatinine > or = 1.8 mg/dl, septic shock, chest X-ray extension, mechanical ventilation, hyponatremia < or = 136 mEq/L, PACO2/FIO2 < 130, and blood urea levels > or = 30 mg/dl were factors related to poor outcome. On the other hand, the following two variables were related to better outcome: adequate treatment for Legionella and pneumonia improvement. The logistic regression analysis demonstrated that APACHE II score > 15 at admission (RR: 11.5; 95% CI 1.75 to 76.1; p = 0.025), and serum Na levels < or = 136 (RR: 21.3; 95% CI 1.11 to 408; p = 0.023), were the only independent factors related to death. On the other hand, improving pneumonia is associated with better outcome in Legionnaires' disease than for patients not having improving pneumonia (RR: 0.019; 95% CI: 0.036 to 0.106; p < 0.0001). A better understanding of the prognostic factors in cases of severe Legionella pneumonia will optimize our therapeutic approach in this disease and help to decrease both its mortality and morbidity rates.