OBJECTIVE To evaluate the tumor extension of extrahepatic bile duct cancer by means of intraductal ultrasonography (IDUS). METHODS IDUS preoperatively assessed the tumor extensions in 25 patients with extrahepatic bile duct cancer. The diagnostic accuracy of IDUS was investigated by comparison with other diagnostic imaging modalities in all cases and with histopathological findings of resected specimens in 18 cases. RESULTS IDUS proved useful in assessing the extension of cancer invasion to the pancreas parenchyma, portal vein, and right hepatic artery. The limitation of the degree of accuracy, based on the group staging criteria, was 68%. IDUS could not assess tumor invasion to the perimuscular loose connective tissue. Therefore, it could not distinguish stage II from stage I. IDUS could not sufficiently assess epicholedochal lymph node metastases (differential diagnosis between stages II and III) and could not demonstrate distant metastases (differential diagnosis between stages IVA and IVB) because of the inevitable attenuation of the echo itself. IDUS could assess cases of stage IVA correctly in 8/8 (100%) cases. The combination of PTC/ERC and IDUS could assess the horizontal extension correctly in 13/18 (72%) cases. The combination of PTC/ERC, percutaneous transhepatic cholangioscopy (PTCS), and IDUS assessed the horizontal extension in 14/15 (93%) cases. CONCLUSIONS 1) IDUS, with a high-frequency probe, was very useful for assessing tumor infiltration in the hepatoduodenal ligament. 2) IDUS could not assess tumor extension outside of the hepatoduodenal ligament, but conventional ultrasonography and angiography could compensate for it. 3) The combination of PTC/ERC, PTCS, and IDUS could assess horizontal extension correctly.