The effect of cyclosporin A in acute and chronically active inflammatory bowel disease was tested in 11 patients with Crohn's disease and two with ulcerative colitis who had exhibited a poor response to at least eight weeks of conventional therapy. Trough levels of the drug in the therapeutic range were obtained in 12 of 13 patients. Cyclosporin A, which was usually added to the continued previous medication, including corticosteroids (11 of 13) or metronidazol (1 of 13), prompted an apparent clinical improvement in all but one patient. In six of the nine Crohn's disease patients with an initial Best index of greater than 150, a definite fall by at least 100 points was observed after 2-10 weeks of treatment, but the van Hees index declined only in two patients. All four patients with chronic perineal fistulation experienced symptomatic relief. Both patients with ulcerative colitis had a clinical remission. Erythrocyte sedimentation rate or serum albumin improved in eight of 13 patients. However, two of the nine responders with Crohn's disease relapsed during cyclosporin A therapy and three immediately after the medication was discontinued. Common side effects included hypertrichosis, tremor, and hyperesthesia; hypertension and epigastric pain each occurred only in one patient.