Objectives: Preoperative evaluation of common duct stones by ERCP is important in the management of gallbladder stones. But, considering the complications and cost of ERCP and the low incidence of choledocholithiasis, it is not necessary to perform ERCP before cholecystectomy in all patients with gallbladder stones. This study was undertaken to propose the indications of preoperative ERCP by predicting common duct stones in patients with gall- bladder stones. Methods: Routine use of ERCP was prospectively studied in all patients with gallbladder stones undergoing elective cholecystectomy. Presence or absence of common duct stones identified by ERCP were analyzed with variable parameters of abdominal pain, fever, number of stones, ultrasonographic criteria of common duct dilatation, and biochemical tests of alkaline phosphatase(ALP), bilirubin, and γ-glutamyl transpeptidase(γ-GTP). Results: A total of 237 patients were included in this study. There was no significant differences between abdominal pain or number of stones and freguency of ductal stones, but incidence of common duct stones was significantly higher in patients with fever than without fever(59.0% vs. 30.7%, p<0.01). There was significantly increased incidence of common duct stones in patients with nondilated ducts(72.5% vs 20.4%, p<0.01), and in patients with abnormalities in any one parameters of ALP, bilirubin, and γ-GTP compared to patient with normal liver function tests(49.7% vs, 7.6%). The incidence of choledocholithiasis was only 1.9% in cases with nondilated duct and normal liver function tests, but the incidence was increased to 20.4%, 33.3%, and 79.0% in cases with nondilated duct and abnormal liver function tests, with dilated duct and normal liver function tests, and with dilated duct and abnormal liver function tests, respectively. Conclusion: The incidence of choledocholithiasis was significantly correlated with presence of fever, liver function tests, and common duct diameter by ultrasonography, but, not with presence of abdominal pain and number of stones. When common bile duct was dilated by ultrasonography and/or liver function tests were abnormal, the probability of ductal stones was high and preoperative ERCP was required to identify choledocholithiasis