Prophylactic metronidazole was compared with no treatment in a randomly allocated study of patients undergoing appendicectomy. A 1-g suppository of metronidazole was given with the premedication and 200 mg t.d.s. orally for 7 days thereafter. There were 4 hospital wound infections in the 87 patients receiving metronidazole and 11 out of 83 in the no-treatment group (P < 0.05). However, 60 per cent of wound infections occurred at home and the overall infection rates were not significantly different. The duration of postoperative hospital stay was longer in the control group (5.8 days +/- 3.0 s.d.) than in the treated group (4.8 +/- 2.3; P < 0.01). Only one anaerobic infection occurred in a patient receiving metronidazole but late infection was not prevented. Blood levels of metronidazole taken at the end of operation showed a wide variation but there was no correlation with subsequent infection. Culture of the appendix showed aerobic organisms in 88 per cent of cases and anaerobes in 72 per cent. It is suggested that an antibiotic which is effective against both aerobic and anaerobic organisms might have an advantage over metronidazole in appendicitis.