Perihilar cholangiocarcinoma is one of the most challenging diseases with poor overall survival. The major problem for anyone trying to convincingly compare studies among centers or over time is the lack of a reliable staging system. The most commonly used system is the Bismuth-Corlette classification of bile duct involvement, which, however, does not include crucial information such as vascular encasement and distant metastases. Other systems are rarely used because they do not provide several key pieces of information guiding therapy. Therefore, we have designed a new system reporting the size of the tumor, the extent of the disease in the biliary system, the involvement of the hepatic artery and portal vein, the involvement of lymph nodes, distant metastases, and the volume of the putative remnant liver after resection. The aim of this system is the standardization of the reporting of perihilar cholangiocarcinoma so that relevant information regarding resectability, indications for liver transplantation, and prognosis can be provided. With this tool, we have created a new registry enabling every center to prospectively enter data on their patients with hilar cholangiocarcinoma (www.cholangioca.org). The availability of such standardized and multicenter data will enable us to identify the critical criteria guiding therapy. (HEPATOLOGY 2011;53:1363-1371 C holangiocarcinoma (CCA) arises from the malignant transformation of the bile duct epithelium; it represents approximately 10% of all primary hepatobiliary cancers and accounts for approximately 2% of all malignancies.1,2 Several lines of evidence indicate that the incidence of CCA has increased over the past 3 decades.3,4 These tumors can develop anywhere along the biliary tree and represent a quite heterogeneous group with distinct patterns, epidemiologies, clinical presentations, and prognoses. The most commonly used classification of CCA has three groups based on the location along the biliary tree: intrahepatic cholangiocarcinoma (IHC); perihilar cholangiocarcinoma (PHC), which is also called a Klatskin tumor 5 ; and distal CCA. The IHC type accounts for less than 10% of the total cases, whereas the PHC type represents about two-thirds of the cases, and distal CCA represents about a quarter of the cases.6 PHC can be defined as tumors that involve or are in close vicinity to the bile duct confluence. We suggest a definition of PHC, which includes tumors above the junction of the cystic duct up to and including the second biliary branches of the right and left bile ducts.The only chance of a cure for this type of cancer is complete surgical resection of the tumor and perhaps liver transplantation in highly select cases. Most of these cancers have a dismal prognosis, and the current 5-year