A prospective, controlled trial of antibiotic therapy was carried out in 90 patients with perforated appendicitis. One randomly selected group received systemic metronidazole, started peroperatively, plus locally instilled ampicillin. The other group, also randomly selected, received only local ampicillin. There was no statistically significant difference in the overall frequency of postoperative septic complications (wound infection or intra-abdominal abscess) between the two groups, but wound infections were significantly fewer in the patients given metronidazole. There was no intergroup difference in hospitalization time. Treatment with systemic metronidazole and local ampicillin is recommended in patients operated on for perforated appendicitis.