In severe appendicitis, the effect of a single intravenous dose of metronidazole (500 mg) was compared with cefoxitin (1 g). The antibiotics were given by random allocation once the diagnosis had been established at operation. In the metronidazole group, 5 out of 48 patients developed a wound infection whilst in hospital compared with 13 out of 48 in the cefoxitin group (P = 0.036). However, 57% of wound infections became apparent after the patient went home and the overall infection rates were similar. The mean length of postoperative hospital stay was 5.0 days in the metronidazole group and 6.8 days in the cefoxitin group (P = 0.052), but of those who did develop a wound infection, the length of stay was almost double in the cefoxitin group. Anaerobic organisms were cultured from the wound in 7 out of 15 patients who received cefoxitin but in none of 5 patients in the metronidazole group. Whilst metronidazole only delayed the discharge of pus from the wound, it did seem to reduce the severity of infection. Cefoxitin appeared to be less effective, given as a single intravenous injection at a dose of 1 g.