The incidence and nature of and the outcome following adverse events were studied prospectively in a surgical intensive care unit over a period of 1 year. From a total of 657 patients, 229 (34.8%) suffered 369 adverse events. The number of adverse events per patient ranged from 1 (58.1%) to a maximum of 4 (6.1%). The overall mortality rate was 23.4%. Eighty-seven deaths (20.3%) occurred in patients not suffering an adverse event and 67 (29.3%) in those whose treatment was complicated by an adverse event (P < 0.02). There was no significant difference in mortality between patients with single or multiple events. Twenty-two patients died as a direct result of the event, the commonest reason being loss of airway control. Adverse events contribute significantly to mortality in critically ill patients.