A prospective, open trial was conducted to test whether i.v. cyclosporine was effective in the treatment of refractory Crohn's disease. Eight patients with acute steroid-refractory attacks were included. Intravenous cyclosporine, 5 mg/kg/day, was added to ongoing drug therapy. Patients who responded were then switched to oral cyclosporine for a mean 2.6-month period, and steroids were discontinued when possible. Six patients improved, with a mean latency time to onset of improvement of 9 days. Two did not improve, and both underwent urgent operation. On oral cyclosporine, five patients maintained remission and discontinued steroids, whereas one relapsed and underwent surgery. After discontinuation of oral cyclosporine, the five remaining patients relapsed, and two underwent surgery. One reversible episode of hepatobiliary toxicity and one of gastrointestinal intolerance were recorded. We conclude that i.v. cyclosporine effectively and rapidly induces improvement of acute steroid-refractory flare-ups of Crohn's disease, but after discontinuation relapse is to be expected.