Diarrhea resistant to many medications and abdominal pain that worsen after meals often interfere with daily life. Malabsorption is an important cause of chronic diarrhea, whereas clinical diagnosis is difficult, especially when using an insurance-covered diagnostic method. In addition, the clinical diagnosis of a digestive disorder, which frequently induces malabsorption, is more difficult. Bile acid malabsorption may cause a digestive disorder, resulting in chronic diarrhea. However, there is no diagnostic method for bile acid malabsorption in clinical practice in Japan. Diarrhea following terminal ileum surgery, cholecystectomy, or upper gastrointestinal surgery also suggests bile acid diarrhea that does not improve with antidiarrheal medications. Bile acids are essential for micelle formation in fat digestion and are reabsorbed at the terminal ileum for reuse after fat absorption. If not reabsorbed and reaching the colon, bile acids activate cyclic AMP-dependent Cl– channels in the colon, moving Cl– ions into the lumen. Na+ ions follow through intercellular spaces, increasing luminal osmolarity and causing water movement into the intestines, resulting in diarrhea. Cholestyramine, an anion exchange resin, binds bile acids by replacing Cl– ions with bile acid COO– ions, excreting bile acids in feces, effectively treating bile acid malabsorption-induced diarrhea. Cholestyramine is useful for diagnostic treatment.