The ERCP report in the patient's chart was compared with findings on common duct exploration or cystic duct cholangiography in 72 patients and found to have a sensitivity of 90.4 percent, a specificity of 98 percent, and an accuracy of 95.8 percent. Factors having the potential to influence the accuracy of ERCP were errors in interpretation by the surgeon and the radiologist and the operative technique of cholecystectomy. Also, the interval between the performance of the procedure and operation was particularly important in the patient with multiple small gallstones or small common duct stones. Small gallstones may spontaneously pass from the gallbladder to the common duct, or small common duct stones may spontaneously pass into the duodenum; therefore, the longer the interval between ERCP and operation, the greater the likelihood of a discrepancy. At operation, gallstones may be squeezed into the common duct during manipulation of the gallbladder unless the cystic duct is obstructed before manipulation of the gallbladder. We found ERCP sufficiently accurate to make cystic duct cholangiography unnecessary in most patients with cholelithiasis having a preoperative ERCP examination.