544 children operated upon for uncomplicated appendicitis were randomised into three groups. Group I received no antibiotics, patients in Group II were administered one single pre-operative dose of metronidazole (10 mg/kg) and patients in Group III received both both metronidazole (20 mg/kg) and cefuroxime (50 mg/kg) in a single preoperative dose. The rate of infectious complications in Group I was 6.3% but only 2.1 and 2.3 in Groups II and III, respectively. Among patients with a normal appendix or patients with simple appendicitis there was no difference in the rate of post-operative infections between the three different groups. However, in patients with gangrenous appendicitis the rate of postoperative infections was significantly lower in the two groups who received preoperative antibiotics (1.8% and 3.0% respectively) than in the group receiving no prophylactic antibiotics (infection rate 14%). It is concluded that a single preoperative dose of metronidazole significantly decreases the rate of postoperative infectious complications in children with appendicitis. No further improvement could be demonstrated if cefuroxime against aerobic organisms was added.