To evaluate the effect of varying durations of antibiotic prophylaxis in trauma patients with multiple risk factors for postoperative septic complications, a prospective randomized trial was undertaken at an urban level I trauma center. The inclusion criteria were full-thickness colon injury and one of the following: (1) Penetrating Abdominal Trauma Index > 25, (2) transfusion of 6 units or more of packed red blood cells, or (3) more than 4 hours from injury to operation. Patients were randomly assigned to a short course (24 hours) or a long course (5 days) of antibiotic therapy. All patients received 2 g cefoxitin en route to the operating room and 2 g intravenously piggyback every 6 hours for a total of 1 day vs. 5 days. Sixty-three patients were equally divided into short-course (n = 31) and long-course (n = 32) therapy. This was a high-risk patient population, as assessed by the mean Penetrating Abdominal Trauma Index (33), number of patients with multiple blood transfusions (51 of 63; 81%), number of patients with an Injury Severity Score greater than 15 (37 of 63; 59%), number of patients with destructive colon wounds requiring resection (27 of 63; 43%), and number of patients requiring postoperative critical care (37 of 63; 59%). Differences in intra-abdominal (1-day, 19%; 5-days, 38%) and extra-abdominal (1-day, 45%; 5-days, 25%) infection rates did not achieve statistical significance. There continues to be no evidence that extending antibiotic prophylaxis beyond 24 hours is of benefit, even among the highest risk patients with penetrating abdominal trauma. A large, multi-institutional trial will be necessary to condemn this common practice with statistical validity.