We conducted a prospective multi-institutional study of nosocomial and community-acquired pneumonias in 1378 patients admitted to the ICUs of six hospitals. We also investigated 1005 of these patients who were intubated and mechanically ventilated for a reason other than pneumonia, the risk of developing pneumonia, and the relationship between the incidence of pneumonia and the length of time during which the patients were mechanically ventilated. A bacteriologic diagnosis was made in 38% of the nosocomial and 21% of the community-acquired pneumonias. The total mortality rate was 40%; 47% of the patients with nosocomial and 17% of the patients with community-acquired pneumonias died. Because it was difficult to make an etiologic diagnosis in two-thirds of the cases, the treatment had to be based on an assumed causative organism.