The Major Trauma Outcome Study (MTOS) is a retrospective descriptive study of injury severity and outcome coordinated through the American College of Surgeons' Committee on Trauma. From 1982 through 1987, 139 North American hospitals submitted demographic, etiologic, injury severity, and outcome data for 80,544 trauma patients. Motor vehicle related injuries were most frequent (34.7%). Twenty-one per cent of patients had penetrating injuries. The overall mortality rate was 9.0%. The mortality rate for direct admissions was strongly related to the presence of serious head injury, 5.0% and 40.0%, when head injuries were less than or equal to AIS (Abbreviated Injury Scale) 3 or greater than or equal to AIS 4, respectively. Survival probability norms use the Revised Trauma Score, Injury Severity Score, patient age, and injury mechanism. Patients with unexpected outcomes were identified and statistical comparisons of actual and expected numbers of survivors made for each institution. Results provide a description of injury and outcome and support evaluation and quality assurance activities.