A method of quantifying the risk of complication following penetrating abdominal trauma is described. A trauma index score was calculated by assigning a risk factor (1-5) to each organ injured and then multiplying this by a severity of injury estimate (1-5). The sum of the individual organ scores comprised the final penetrating trauma index (P.A.T.I.). A consecutive series from 1975 to 1979 of 108 isolated stab wounds and 114 gunshot wounds to the abdomen requiring laparotomy were analyzed by this scheme. Only patients surviving 24 hours postoperatively were evaluated. Gunshot wounds resulted in P.A.T.I. scores greater than 25 in 39 (34%) of the patients. This was associated with a 46% complication rate and contrasted to a 7% incidence when the P.A.T.I. was equal to or less than 25. Following abdominal stab wounds six (6%) of the patients had P.A.T.I. scores exceeding 25. Complications developed in 50% of this group compared to 5% when the P.A.T.I. was less than 25. Using the P.A.T.I., we conclude that gunshot victims are more likely to need trauma center treatment than those with stab wounds. The P.A.T.I. also might facilitate cost analysis of trauma care. The ability to assess effects of patient age, sex, amount of blood loss, duration of hemodynamic shock, and metabolic response to injury might further enhance the therapeutic implications in both penetrating and blunt abdominal trauma.