Thirty-eight patients with external bile fistula created operatively for choledocholithiasis or malignant tumors were subjected to measurement of biliary mannitol clearance as an indicator of canalicular bile flow. The clearance measurement was performed by the single injection method in 29 cases and the constant infusion method in 12 cases, either of which consisted of administering mannitol intravenously and then directly measuring its concentrations in the plasma and the bile by Tibbling's method. The clearance value of either method was in good agreement with each other, and the bile fractions thus obtained were almost equal to those obtained from the clearance measurement done by Boyer, using radioisotope. The bile fractions were measured in 8 patients with T-tube on the 2nd and 16th post-operative days. Ductal bile flow was easy to vary, accelerated in cholangitis or bile duct dilatation and decreased as the infection subsided or the obstruction was relieved, while canalicular bile flow steadily increased postoperatively. However, of the patients given biliary drainage for obstructive jaundice due to malignant tumors, 4 cases with persistent postoperative hyperbilirubinemia had the reduced canalicular flow. Three other patients presenting with excessive bile output showed not only low canalicular flow but also high ductal flow.