Background. Patients with idiopathic bile acid diarrhea (IBAD) have accelerated transit in the small bowel and in the colon. This study evaluated the effect of cholestyramine, a bile acid binder, on transit in the small bowel and in the colon. Methods. Thirteen subjects with IBAD (4 male, 9 female; aged 23-67 years) were included. Gastrointestinal transit was investigated in all subjects before and during treatment with cholestyramine. Results. Stool frequency was reduced (P < .05), and stool consistency was harder (P < .01) during treatment. A tendency to prolonged small bowel transit time was shown (P = .05). Transit in the transverse colon was prolonged by treatment (P < .02). Conclusions. A bile acid binder is an effective treatment in IBAD, probably by reducing transit in both the small bowel and in the colon. The results also indicate that the small bowel as well as the colon has a pathophysiological role in IBAD.