The serum 25-OHCC concentration was measured in 151 patients in order to evaluate the potential use of this determination in the management of gastrointestinal disorders. Patients with functional bowel disease had lower serum mean 25-OHCC levels than normals. The results were divided into normal (greater than 21 ng/ml), low normal (12-21 ng/ml), and low levels (less than 21 ng/ml). Two thirds of patients with malabsorption had low serum 25-OHCC. Most patients following jejunoileal bypass surgery for morbid obesity had low levels despite supplemental oral calciferol therapy. In patients with chronic liver disease, cholestasis more than parenchymal cell disease appeared responsible for low serum 25-OHCC levels. Measurement of serum 25-OHCC may be an ancillary screening test for fat malabsorption and in patients with intraluminal bile salt deficiency. Furthermore, repeated measurements may be useful in monitoring therapy with vitamin D products in patients with chronic malabsorption and cholestasis.