One hundred patients undergoing cholecystectomy underwent ultrasonography of the biliary tree on the day prior to surgery. At operation a per-operative cholangiogram was performed unless stones were palpable in the duct. Pre-operative biliary ultrasonography accurately identified dilatation of the common bile duct (sensitivity 96%, specificity 95%) but was less accurate at detecting common duct stones (sensitivity 36%, specificity 98%). Thirty three percent of patients with dilated ducts on ultrasound did not have stones in the duct, while 20% of patients with common duct stones had normal sized ducts. We conclude that ultrasonography alone cannot reliably select patients who require exploration of the common bile duct, or select patients for operative cholangiography. Although pre-operative demonstration of common bile duct dilation is an absolute indication for operative cholangiography, by itself it does not indicate the need for exploration.