Background and Aims: Interleukin 10 (IL-10) is a cytokine with immunosuppressive and anti-inflammatory activities. Gene-targeted IL-10- deficient mice develop a chronic intestinal inflammatory disease that is reminiscent of Crohn's disease. The present double-blind randomized multicenter trial was designed to evaluate the safety, tolerance, and pharmacokinetics of IL-10 in Crohn's disease. Methods: Forty-six patients with active steroid-resistant Crohn's disease were treated with one of five doses of recombinant human IL-10 (0.5, 1, 5, 10, or 25 μg/kg) or placebo administered once daily by intravenous bolus injection over 7 consecutive days. Results: Treatment was safe and well tolerated, and no evidence for IL- 10 accumulation was observed at the end of the treatment period. At the end of the study, Crohn's disease activity scores were 179 in IL-10-treated patients and 226 in patients receiving placebo. The proportion of patients that experienced a complete remission at any time in the 3-week follow-up period was 50% in the 11-10 group and 23% in placebo-treated patients. Conclusions: These results indicate that IL-10 administered as a daily bolus injection over 1 week is safe and well tolerated and may be clinically efficacious.