By oral administration of chenodeoxycholic acid (CDCA) varying in dose for 7 consecutive days to patients who underwent cholecystectomy with biliary drainage for cholesterol gallstone, the biliary cholesterol dissolubility was studied in pursuit of the optimum dose in CDCA therapy for cholesterol gallstone. The rate of dissolution of biliary cholesterol by CDCA administration was highest in cases of 250 mg/day in dose, followed by those of 750 mg/day. When the dose was observed from per kg of body weight, the dissolubility of cholesterol was highest in the case of 6.3 mg/kg, followed by one of 12 mg/kg. Thus, it is presumed that in CDCA therapy for cholesterol gallstone a dose of 250 mg/day is too small and 750 mg/day too large in amount from the viewpoint of its effectiveness. The optimum dose probably lies between 5 mg/kg and 12 mg/kg. A transient elevation in levels of SGOT and SGPT was observed only in one case treated with 750 mg/kg.