Three patients are presented with a history of chronic watery diarrhea due to bile acid malabsorption, proved by the tauro-23[75Se]selena-25 homocholic acid test and an unequivocal response to cholestyramine therapy. Fecal fat tests, Schilling tests, and barium studies of the small intestine and colon were all normal. Jejunal biopsies were normal but multiple biopsies of the terminal ileum, performed by retrograde ileoscopy, showed uniform crypt hyperplastic villous atrophy and features of colonic metaplasia with increased mononuclear infiltration of the lamina propria. All 3 patients demonstrated evidence of abnormal immune function, including the presence of serum autoantibodies, circulating immune complexes, and hypocomplementemia. One patient had Sjögren's syndrome. This disorder, which might be immunologically mediated, should be called primary bile acid malabsorption and should be distinguished from other ileal disorders.