All patients with severe pneumonias (community-acquired and nosocomial) who required treatment in the intensive care unit (ICU) were included in a 3-year prospective study. Predictive factors for a fatal outcome were analyzed in 127 patients. An etiologic diagnosis was made in 70 (55.1%) patients. Culture of sputum or tracheobronchial secretions were used only as criteria for microbiologic diagnosis of Legionella pneumophila. The pathogens most frequently identified were L pneumophila, Streptococcus pneumoniae, and Pseudomonas aeruginosa. Viruses were not detected as causative agents. A total of 54 patients died (mortality rate, 42.5%). The univariate analysis showed the following factors associated with mortality: advanced age (≥70 years); presence of septic shock, requirement of mechanical ventilation, and Simplified Acute Physiology Score [SAPS] index >12 at the time of admission to the ICU or when symptoms appeared in patients already admitted to the ICU; development of any complication during ICU hospitalization; and P aeruginosa as the etiologic agent of the pneumonia. When all variables were introduced by a stepwise method, the final model included advanced age (≥70 years), SAPS index > 12, presence of septic shock, requirement of mechanical ventilation, bilateral pulmonary involvement, and P aeruginosa as the etiologic agent of pneumonia as prognostic factors associated with a fatal outcome.