The effect of cyclosporin was evaluated in six patients with severe ulcerative colitis not responding to at least 8 days of standard therapy with intravenous corticosteroids. Cyclosporin (5-7.5 mg/kg/day intravenously) was added while steroid therapy was continued. Five of 6 patients responded after a mean of 7 days and colectomy was not necessary. After 4 weeks three patients achieved clinical remission or had mild symptoms and were weaned from cyclosporin and corticosteroids without exacerbation within the next 7-15 months. Two patients improved and they were put on oral cyclosporin. One of them relapsed after 2 weeks and then responded to high dose corticosteroids. This patient is doing well at 8 months of followup on azathioprine and steroids. One patient stopped oral cyclosporin after 3 months abruptly and then had a relapse. He subsequently improved while refusing any medical therapy. Side effects of cyclosporin occurred in 2 patients but were mild and self limited and did not necessitate discontinuation of the drug. Cyclosporin appears to be effective in a large portion of patients with severe ulcerative colitis who failed to improve on corticosteroids and in whom colectomy would otherwise be considered.