Purpose : The purpose of this study was to evaluate the utility of the CT cholangiography by using helical CTwith multiplanar reformation in patients with extrahepatic biliary obstruction. Materials and Methods : Helical CTwas performed in 36 patients of extrahepatic biliary obstruction confirmed by operation or invasivecholangiography(percutaneous transhepatic cholangiography or endoscopic retrograde cholagiopancreatography). Thecause of obstructions were 18 benign diseases(11 common bile duct stones, four choledochal cysts, threepancreatitis) and 18 malignant diseases(seven common bile duct carcinomas, seven pancreatic head carcinomas, threeampullary carcinomas, one periampullary duodenal carcinoma). After helical scanning through the extrahepatic bileduct, retrospective reconstruction of the helical data was performed. And then, CT cholagiogram was obtained withmultiplanar reformation. We evaluated the technical success rate and the accuracy in determinating the level andthe cause of the obstruction. We compared findings of the CT cholangiogram with that of operation or invasivecholangiography. Results : In 100%(36/36) of cases, CT cholangiography could be obtained successfully. Theaccuracy of the CT cholangiography in determinating the level of the obstruction was 100%(11 cases ofsuprapancreatic duct, 13 cases of intrapancreatic duct, 13 cases of infrapancreatic duct and ampulla), and theaccuracy in determinating the cause of the obstruction was 91.7%(all cases of 18 benign diseases, and 15 cases of18 malignant diseases). Conclusion : In evaluating the obstruction of extrahepatic bile ducts, the CTcholangiography by using helical CT with multiplanar reformation is an useful noninvasive method in determinatingthe level and the cause of biliary obstruction and therefore could replace the invasive cholangiography.