Trauma Score (TS), APACHE II score, and Injury Severity Score (ISS) have been utilized to quantitate severity of illness in various groups of patients. The purpose of this study was to compare the relationship of the APACHE II score, TS, and "computer-derived" ISS with outcome in critically injured trauma patients. Data were recorded prospectively in a computer database for 428 consecutive trauma admissions. Stepwise discriminate analysis was utilized to determine the best predictor of both intensive care unit (ICU) and hospital outcome. Forty-two patients died in the ICU (10%), and another 18 patients died after leaving the ICU (4%), for a total mortality rate of 14%. The mean p value and partial R2 value obtained from stepwise discriminant analysis of the relationships between APACHE II score, TS, and ISS to ICU and hospital survival are shown. APACHE II score was the best predictor of both ICU and hospital outcome in these critically ill trauma patients. However, when combining all three measures (APACHE II score, TS, and ISS), only a portion of the variance in outcome is explained by the scores (R2 < 0.05). We conclude that scoring systems for outcome prediction should be utilized only as an adjunct to clinical assessment in the evaluation of the severity of illness and mortality risk in critically ill trauma patients.