Although bile acid malabsorption (BAM) in post-cholecystectomy diarrhea (PCD) is a well-known clinical condition, its true etiopathogenetic role is not entirely clear. The SeHCAT (23-selena-25-homotaurocholic acid) test, a simple and reliable BAM test, was performed in 33 cholecystectomized patients, 26 with chronic diarrhea. The test revealed a marked degree of BAM in 25/26 cases. Cholestyramine in doses of 2-12 g/day was effective in 23/25, ineffective in two, and was not tolerated in one patient. When treatment was suspended, diarrhea recurred in nine, whereas bowel habit remained regular in 60%, with brief sporadic episodes of diarrhea in the other cases. The SeHCAT test was repeated in 11 cases after cholestyramine treatment interruption, and revealed the normalization of parameters in two patients and an improvement in three. We conclude that BAM is an important etiopathogenetic factor in PCD that responds favorably to cholestyramine. In 60% of the cases, it resolved diarrhea definitively, although without eliminating BAM in all cases: this suggests that existence of other factors associated with BAM. The SeHCAT test is essential for a differential diagnosis between PCD and the irritable bowel syndrome.