We examined the incidence of "missed" biliary calculi in a teaching center where intraoperative biliary endoscopy is performed routinely. During a six-year period, 1,280 procedures were performed for benign biliary disease. Two hundred eight patients (16%) underwent common duct explorations with choledocholithiasis discovered in 152 patients (73%). Choledochoscopy, using a rigid endoscope equipped with the Hopkins rod-lens system, was performed in 148 patients. When the scope was used, missed stones were seen on a follow-up T-tube cholangiogram in only 2.3% of the patients undergoing choledocholithotomy. More than 90% of the procedures were performed by supervised resident surgeons who had had no specialized training in biliary endoscopy. The incidence of retained stones after choledocholithotomy and choledochoscopy was lower than those of other reported series in which the choledochoscope was not used. We recommend routine use of intraoperative biliary endoscopy in all patients who undergo common duct exploration.