SummaryBile acid malabsorption in cystic fibrosis reduces intraluminal bile acid concentration and may impair fat absorption. The cause of this malabsorption is unknown but it is believed due to intraluminal inhibition of uptake by undigested dietary nutrients. The purpose of this study was to determine the bile acid absorptive capability of cystic fibrosis intestine in a physiologic environment. Direct ileal mucosal taurocholic acid uptake was examinedin vitroin seven patients with cystic fibrosis, and seven children and adolescents with ileostomies as controls. Jejunal uptake was studied in five normal individuals. A Crosby-Kugler biopsy capsule was used to obtain all tissues. Tissue was incubated in Krebs buffer, 10 mM glucose, and taurocholic acid at 0.1, 1.0 and 10.0 mM with shaking at 37°C. Significant reduction of taurocholic acid uptake was present in every cystic fibrosis patient with mean uptake rates being 24%, 38%, and 29% of control ileum, respectively, at the three concentrations. Values paralleled those for passive jejunal taurocholic acid uptake in controls. These data illustrate a marked reduction in taurocholic acid uptake capability of cystic fibrosis ileal mucosa and may indicate a cellular defect suggestive of a primary lesion in this disease.