Intrahepatic cholangiocarcinoma (ICC) is a primary malignancy of the liver second in prevalence only to hepatocellular carcinoma. 1 In East-Asian countries where hepatolithiasis is not uncommon, ICC is known to develop in patients with hepatolithiasis: the incidence of ICC in hepatolithiasis in Taiwan and Japan is approximately 5%. [2][3][4] ICCs, at least those arising in hepatolithiasis, are known to show multistep progression, undergoing a biliary dysplasia-carcinoma sequence. 1 With recent advances in imaging technology, ICC can now be detected relatively early and is surgically resectable. Surgically resectable ICCs are divided into 3 gross types (massforming [MF], periductal-infiltrating [PI], and intraductalgrowth [IG] types). 5 The IG type is the least common and shows a predominant intraductal growth with no or mild extension beyond the bile duct walls. Recently, ICC of the IG type has received attention in Asian countries. Suh et al. 6 reported 16 cases among 122 surgically resected ICCs (14.3%) in Korea, and that such cases had a better prognosis after complete surgical resection. In Japan, Ohashi et al. 7 reported that 6 of 72 surgically resectable cases of ICC were of the IG type. Previously used terms such as mucin-producing ICC, intraductal mucosal-spreading mucin-producing peripheral cholangiocarcinoma, intraductal growth type of peripheral cholangiocarcinoma, and intraductal variant of peripheral cholangiocarcinoma, 8-10 describe neoplasms similar to or identical to the IG type. Although there have been a number of clinical studies on this type of ICC, the histogenesis and progression of this type remains speculative.Biliary neoplasms arising in hepatolithiasis present occasionally with intraductal papillary carcinoma with and without variable periductal invasion, and intraductal papillary proliferation with variable dysplasia is also encountered in hepatolithiasis. 6,11 To date, the histogenesis, histopathology, and early and advanced lesions of this tumor have not been explored in detail.In this study, we examined a total of 62 cases of hepatolithiasis with intraductal papillary growth of dysplastic and carcinomatous biliary epithelia of the intrahepatic biliary, including mucin-producing ICC in Taiwan, and described the histopathologic features with respect to the background biliary pathology and also in comparison with hepatolithiasis cases in Japan. MATERIALS AND METHODS Classification of the Intrahepatic Biliary TreeThe intrahepatic biliary tree is distal to the right and left hepatic duct and is further classified into intrahepatic large bile ducts, septal bile ducts, interlobular bile ducts, and bile ductules. 12 Intrahepatic large bile ducts are recognizable grossly and are continuous with the intrahepatic peribiliary glands. Septal bile ducts are microscopically identifiable and are composed of columnar epithelial cells and thick fibrous bile duct walls (internal diameter Ͼ100 m). Interlobular bile ducts are composed of low-columnar or cuboidal epithelial cells, but lack periductal fi...