The function of the choledocho-duodenal sphincter was studied in 16 patients, 8 with juxta-papillary duodenal diverticula. All patients had calculi in the gallbladder. The common bile duct was normal. At cholecystectomy two catheters were introduced into the common bile duct through the stump of the cystic duct and fixed in place. The examinations were performed when the patients had recovered from the operation. One catheter was connected to a pressure transducer, the other was used for saline infusions. Pressure in the common duct was recorded before infusion and at constant infusion rates of 3,6, and 12 ml/min. The muscular tone, the contractile activity, and the total rhythmic variations of the sphincter during infusions were all significantly less in patients with diverticula than in the controls without diverticula. The findings indicate that there is a dysfunction of the choledochoduodenal sphincter in patients with juxta-papillary duodenal diverticula. This may in part be responsible for the high incidence of biliary calculi in patients with duodenal diverticula.