We investigated the safety, clinical efficacy, and significance of tacrolimus trough levels in 20 outpatients with active ulcerative colitis. Adverse effects were observed in 16 (80%) patients, and most events were reversible and showed improvement or complete recovery. Three patients required hospitalization because of disease aggravation. The rate of efficacious tacrolimus trough levels was 60% within 1 week and 95% within 2 weeks. At 12 weeks, the clinical remission rate was 25% and the improvement rate was 55%. The clinical remission rate was significantly higher in patients with the relapse-remission type than in those with the chronic inflammation type. These results indicate that the use of tacrolimus in outpatients with ulcerative colitis is safe and that efficacious trough levels can be rapidly achieved in these patients.