The purpose of this prospective clinical study was to determine the incidence, bacteriology and outcome of lower respiratory tract infections developed among 208 consecutive, critically ill comatose patients, hospitalized in a university hospital, medical-surgical intensive care unit, over a three year period. Nosocomial pneumonia developed in 53 (25%) patients after a mean of 8.1 days (range 3-31 days). Furthermore, there were five superinfections, raising the mean incidence to 28%. One patient developed secondary bacteraemia, and another two had septic shock. Fifteen (28%) patients from the pneumonia group died, and six of these deaths were considered to be directly related to nosocomial pneumonia. Identification of the causative agent, using the protected specimen brush technique, was possible in 42 episodes; in 10 (24%) of these cases more than one microorganism was isolated. Gram-positive cocci represented 53% of isolates, and Staphylococcus aureus accounted for 78% of all Gram-positive cocci, being the most frequent microorganism in this population. In conclusion, nosocomial pneumonia is a common complication of critically ill comatose patients. Its characteristic aetiological spectrum in this population should affect antibiotic prescription. Consequently, we suggest including antimicrobial drugs which are active against S. aureus in the empirical regimen until aetiological diagnosis is established.