The pool size and composition of bile acids were studied in 13 unoperated patients with uncomplicated Crohn's disease; 10 patients with ulcerative colitis, and 10 normal subjects. Many patients with Crohn's disease had in their bile a significantly increased amount of ursodeoxycholic acid. The bile acid pool size was significantly decreased and the ratio of glycine to taurine conjugates was significantly increased in the Crohn's disease patients. The reduction in bile acid pool size was related to the activity of the disease. The disorders of bile acid metabolism suggest that the intestinal involvement in Crohn's disease is much more extensive than can be demonstrated by careful radiological examinations.