Intravenous cyclosporin (4 mg/kg daily) was given to fifteen patients with severe active ulcerative colitis, fourteen of whom had failed to respond to at least 10 days' treatment with intravenous steroids. In the acute phase of the open trial, eleven patients (73%) improved and avoided colectomy; the mean response time was 5.8 days. Patients who responded were maintained on oral cyclosporin (6-8 mg/kg daily) for a 6-month chronic phase. Six of the eleven patients have been weaned from steroids and remain in clinical and endoscopic remission 5-18 months after discontinuation of cyclosporin; three are well at 5 months and their steroid dose is being tapered; one patient relapsed after 5 months on oral cyclosporin and underwent colectomy; and one patient failed to show continued improvement with oral cyclosporin but is in clinical remission after changing to mercaptopurine treatment. Slight self-limiting adverse side-effects of cyclosporin were seen, but none necessitated withdrawal from the study. Cyclosporin seems to be an effective treatment for patients with severe active ulcerative colitis who have not responded to steroids.