Bile acid malabsorption was measured as the fecal excretion of 14C after intravenous administration of carboxyl-14C-cholic acid and was studied in 31 patients, 8-17 years old, with chronic colitis. There were 15 patients with proven or probable Crohn's disease and 16 patients with ulcerative colitis. The mean excretion was 14.7% and 19.9%, respectively. The patients with moderate or severe inflammatory activity in the ascending colon, assessed by colonoscopy, had a significantly higher 14C excretion than the patients with no or mild inflammatory activity, 24.1% and 6.6%, respectively. These findings suggest that the ascending colon is more important in the preservation of bile acids than has usually been postulated. When the inflammation in this part of the colon is pronounced, the 14C excretion is high, independent of the type of colitis and the clinical disease activity.