Background and Aim:  In patients with hilar cholangiocarcinoma (HC), longitudinal tumor extent is important for curative resection. The purpose of this study was to evaluate the longitudinal extents of HC using transpapillary intraductal ultrasonography (IDUS) for optimal surgical planning. Methods:  From July 2006 to April 2010, a total of 42 patients with borderline resectable HC were enrolled at Samsung Medical Center, a tertiary referral hospital in Seoul, Korea. All patients were evaluated using multi‐detector computed tomography (MDCT), endoscopic retrograde cholangiopancreatography (ERCP) and IDUS. The new modified Bismuth Type (MBT) classification, in which the traditional Bismuth type IV stage is divided into stages IVa and IVb, was used to determine whether the tumor invaded the left lateral section. Among the subtypes of HC, the periductal infiltrative type (PDI) and intraductal papillary neoplasm of the bile duct (IPN‐B) were compared. The accuracies of CT, ERCP, and IDUS were assessed through comparison with the postoperative histology findings. Results:  A total of 42 patients were prospectively enrolled, and the tumor extent could be assessed histologically in 30 patients. The accuracies of CT, ERCP, and IDUS were 66.6%, 60%, and 90%, respectively. The accuracy of IDUS was 85.7% in 21 patients with PDI and 100% in nine patients with IPN‐B. Conclusions:  Hilar cholangiocarcinoma staging based on IDUS findings was highly accurate. We therefore highly recommend using IDUS for optimal surgical planning in patients with borderline resectable HC.