Hospital-acquired pneumonia was studied prospectively for 3 1/2 years in a 549-bed facility with acute medical-surgical care wards, convalescent wards, and a chronic care unit. Bacteriological studies were limited to transtracheal aspirates, pleural fluid, and blood cultures. The predominant isolates in 159 patients were gram-negative bacilli (47%), anaerobic bacteria (35%), Staphylococcus aureus (31%), and Streptococcus pneumoniae (26%). Nearly half of all specimens yielded a polymicrobial flora with more than one potential pathogen. Distribution of pathogens was similar with analysis of all patients, including patients with a monomicrobial infection and patients with bacteremic pneumonia. The prevalence of cases and distribution of bacteria were similar for patients located on acute medical-surgical wards and those in the nursing home care unit. Nosocomial pneumonia was judged directly responsible for lethal outcome in 19% of patients and a contributing factor to death in another 13%.