Nosocomial infections are a major problem in intensive care patients. Thirty-nine patients, requiring intensive care for 5 days or more (mean 15.8 days) were prospectively investigated, to determine the relation between colonisation and nosocomial infection. Thrice weekly, cultures from the oropharynx, respiratory and digestive tract were obtained. Colonization with aerobic gram-negative microorganisms of the oropharynx, respiratory and digestive tract significantly increased during the stay in the Intensive Care Unit. In 29 patients (74%) 78 nosocomial infections were diagnosed. The most frequent nosocomial infections were pneumonia (26 patients, 66.6%), catheter-related bacteraemia (11 patients, 28.2%), and wound infections (7 patients, 17.9%). In 59 instances (75.6%), colonization with the same potential pathogenic microorganism preceded the nosocomial infection. The overall mortality was 25.6% (10 patients), bacteraemia with aerobic gram-negative microorganisms being the cause of death in 7 patients.