Ileal Na+-dependent bile acid transport was quantified in vitro as the uptake of 3H-taurocholate into brush-border membrane vesicles. Vesicles were prepared from ileal biopsies of 158 patients placed in 10 diagnostic categories. Active bile acid transport (expressed as picomoles taurocholate uptake per milligram brush-border membrane protein per 15 s, median and interquartile ranges indicated) did not differ significantly in 6 categories: irritable bowel syndrome (71, 35-97; n = 21), colon polyps (42, 30-89; n = 29), colitis (62, 33-91; n = 31), postvagotomy or postcholecystectomy (69, 37-97; n = 11), diarrhea without increased bile acid loss (58, 48-85; n = 12), and lack of gastrointestinal pathology (74, 45-103; n = 22). A decreased active bile acid transport was found in 3 categories: ileal disease (4, 1-36; n = 11), partial ileal resection (5, 1-35; n = 5), and constipation (41, 22-50; n = 8). Bile acid transport was increased in patients with bile acid-losing diarrhea with endoscopically and histologically normal ilea (111, 94-135; n = 8). These findings indicate that a low fecal bile acid loss, presumed to be present in constipated patients, is associated with a low Na+-dependent ileal bile acid transport and a high bile acid loss is associated with a high active bile acid transport. Ileal bile acid transport might be regulated by the availability of bile acids to the ileal enterocytes.