Hilar cholangiocarcinomas involving the bifurcation of the hepatic duct are called Klatskin tumors. A resected specimen of the hilar hepatic region with Klatskin tumor was analyzed. The lining epithelium of major biliary ducts was regular, while the majority of epithelial cells lining the excretory ducts of peribiliary glands (PBGs) exhibited malignant features. The connective tissue surrounding the PBGs was infiltrated by mucinous malignant epithelial cells, sometimes in a signet-ring cell form, with perineural invasion. The tumor epithelial cells showed distinct CK 7 and CA 19-9 positivity. The described cholangiocarcinoma was classified as the Bismuth-Corlette type IIIb and originated from the excretory ducts and acinar cells of PBGs.