Recent studies suggest that cyclosporine may be an alternative to colectomy in children with severe ulcerative colitis who fail traditional therapy with bowel rest and high-dose corticosteroids. We report the clinical course of two such children. Patient 1 received oral cyclosporine at 15 mg/kg/day for 1 week followed by 8 mg/kg/day for 6 weeks. Clinical remission occurred within 5 days and has been sustained for 24 weeks after azathioprine was substituted for cyclosporine. Patient 2 received oral cyclosporine at 7-8 mg/kg/day for 7 days after which colectomy was performed due to lack of clinical improvement. Blood cyclosporine levels for patient 1 ranged from 95 to 406 ng/ml and for patient 2 from 36 to 65 ng/ml. Sigmoid colonic tissue cyclosporine concentrations for patients 1 and 2 were 10,058 and 3,205 ng/g, respectively. The patient who responded had significantly higher blood and colonic tissue cyclosporine concentrations than the patient who did not respond. Further studies with larger numbers of patients are needed to determine if there is a correlation between blood and colonic tissue cyclosporine concentrations and clinical response.