A prospective, randomized study was undertaken in 246 patients 4-15 years of age with simple or complicated appendicitis and local peritonitis to determine the efficacy of oral metronidazole (OM) therapy. Those referred on odd days were chosen as the study group (SG) and those on even days as a control group (CG). The SG received OM 10 mg/kg per dose 2 h preoperatively and every 8 h after operation according to the following intraoperative findings: inflamed (mild to severe), 3 doses; gangrenous or perforated with no pus, 3 days, the same with pus, 5 days. The CG received 20 mg/kg cephalexin 2 h before operation and, if the appendix was inflamed, 6-hourly for 3 doses postoperatively. The routine combination in our center of penicillin, chloramphenicol, and gentamicin was given to the patients in the CG with complicated appendicitis, as in the SG, for 3 or 5 days. All cases with generalized peritonitis were excluded from the study. Serum concentrations of metronidazole after one postoperative dose were in the bactericidal range in 18 of 20 patients in whom the measurement was performed, The incidences of wound infection and intraabdominal abscess were quite similar in both groups with the same degree of pathology. However, in patients with complicated appendicitis the hospital stay in the SG was about 1 day less than in the CG. Moreover, hospital costs per patient day were less in the SG. We conclude that OM is a cost-effective drug, more convenient for the patient and nursing staff, and can be used as an effective antibiotic even in complicated appendicitis.