A prospective double blind controlled trial was undertaken to examine the role of metronidazole as an adjunct to corticosteroids in the management of severe ulcerative colitis. Thirty nine patients with severe ulcerative colitis were randomised on admission to hospital to receive either intravenous metronidazole 500 mg eight hourly (19 patients) or an identical intravenous placebo (20 patients). The two groups were similar with respect to age, sex, and the extent of colitis. In addition all patients received a standard intravenous regimen consisting of methyl prednisolone 16 mg six hourly and parenteral nutrition together with a twice daily hydrocortisone 100 mg enema. Treatment was continued for five days when the patients were formally assessed. Fourteen of 19 patients (74%) receiving metronidazole and 14/20 (70%) receiving placebo were substantially improved, or in remission at the end of five days. Five patients treated with metronidazole and six with placebo had no improvement and all proceeded to urgent colectomy with no operative mortality. There were three late deaths, one in the metronidazole and two in the placebo group. These results do not support the routine use of intravenous metronidazole in the treatment of severe ulcerative colitis.