Bile acid and vitamin B12 malabsorption were evaluated in 34 cases of ulcerative colitis. Twenty-four patients were non-operated and 10 patients were colectomized. The postprandial duodenal bile acid concentration was abnormally low in 13 of 24 non-operated cases and found to be correlated to the activity of the disease. Two of six patients subjected to colectomy had a reduced bile acid concentration. Bile acid absorption was assessed by the cholyl-glycine-1-14C breath test combined with faecal analysis. The 14C-excretion in breath was abnormally elevated in only one of the patients in the total material. The faecal 14C-output was related to the disease activity in the non-operated group. Patients colectomized for ulcerative colitis had an extremely high excretion of isotope in the ileal effluent, from 15 to 81 per cent of the dose given. The faecal 14C-output was correlated with the duration of the ileostomy and the mass of ileal discharge. Vitamin B12 malabsorption was only present in five patients. It is concluded that patients with ulcerative colitis during the active phase of the disease have bile acid malabsorption, and patients colectomized for ulcerative colitis have an abnormal high bile acid deconjugation in the ileal effluent.