ASCOT (A Severity Characterization of Trauma) is a physiologic and anatomic characterization of injury severity which combines emergency department admission values of Glasgow Coma Scale, systolic blood pressure, respiratory rate, patient age, and AIS-85 anatomic injury scores in a way that obviates ISS shortcomings. ASCOT values are related to survival probability using the logistic function and regression weights reaffirm the importance of head injury and coma to the prediction of patient outcome. The ability of TRISS and ASCOT to discriminate survivors from non-survivors and the reliability of their predictions, as measured by the Hosmer-Lemeshow statistic, were compared using Major Trauma Outcome Study (MTOS) patient data. ASCOT performance matched or exceeded TRISS's for blunt-injured patients and for penetrating-injured patients. ASCOT performance gains were modest for blunt-injured patients. The Hosmer-Lemeshow statistics suggest that ASCOT reliably predicts patient outcome for penetrating-injured patients and nearly so for blunt-injured patients. Statistically reliable predictions were not achieved by TRISS for either set. ASCOT provides a more precise description of patient physiologic status and injury number, location, and severity than TRISS. The ASCOT patient description may be useful in relating to other important outcomes not highly correlated with TRISS or the Injury Severity Score (ISS) such as disability, length of stay, and resources required for treatment.