Over a period of 4 consecutive yr, 92 nonimmunosuppressed patients (21 women and 71 men aged 53 ± 16 yr, ± SD) with critical acute respiratory failure (Pa_O2/Fi_O2, 209 ± 9 mm Hg) caused by severe community-acquired pneumonia were admitted to the respiratory intensive care unit (RICU) of a general hospital. The most frequent underlying clinical condition was chronic obstructive pulmonary disease (44 patients, 48%). A total of 56 patients (61%) required mechanical ventilation for a mean period of 10.7 ± 12.5 days, 29 of them (52%) needing PEEP (9.9 ± 3.8 cm H₂O). A group of 23 (25%) patients had criteria of adult respiratory distress syndrome (ARDS). A causal microorganism was identified in 48 patients (52%), the two most frequent etiologies being Streptococcus pneumonias (14, 15%) and Legionella pneumophila (13, 14%). Pseudomonas aeruginosa (5, 5%) was always associated with bronchiectasis. Mortality due to severe community-acquired pneumonia was 22% (20 patients). According to univariate analysis, mortality was associated with anticipated death within 4 to 5 yr, inadequate antibiotic treatment before RICU admission, mechanical ventilation requirements, use of PEEP, Fi_O2 > 0.6, coexistence of ARDS, radiographic spread of the pneumonia during RICU admission, septic shock, bacteremia, and P. aeruginosa as the cause of the pneumonia. Further, recursive partitioning analysis selected two factors significantly related to the prognosis: the radiographic spread of the pneumonia during RICU admission and the presence of septic shock. Our statistical analysis could accurately predict prognosis in 75 patients (82%). Recommendations for antibiotic chemotherapy in these patients with both community-acquired pneumonia and severe acute respiratory failure are suggested.