Lecture : Intraductal papillary neoplasm of the bile duct (IPNB) is a rare disease with reported prevalence of 4–38% of all bile duct tumors and has a geographical prevalence in Eastern countries. It was first introduced in 2006 by Zen et al. and adopted as a distinct tumor entity in the 2010 World Health Organization (WHO) classification8 which is characterized by dilated bile ducts filled with a non-invasive papillary or villous biliary neoplasm covering delicate fibrovascular stalks. Since it had been called by various names before 2012, such as biliary papillomatosis, biliary intraductal papillary-mucinous neoplasm, mucin-producing cholangiocarcinoma, and mucin-hypersecreting bile duct tumor, many previously published articles included all heterogeneous cases that have some similarities in intraductal growth pattern and various mucin production. Due to the rarity of disease and non-standardization of clinico-pathological definition and classification, we need collaboration study on IPNB between Korea and Japan that have reported over 50% of papers on IPNB in the world. The aim of this study is to explore the clinicopathologic features and establish the definition of pathologic subtype of IPNB based on the large multicenter cohort. . For this Japan-Korea collaborative, retrospective study, patients who had undergone surgery for IPNB were collected from 67 institutions, 44 in Japan and 23 in Korea. After the central review of pathology, two study subjects were investigated in Korea and Japan. (1) Clinicopathologic analysis of intraductal papillary neoplasm of bile duct: Korean multicenter cohort study Among 587 patients previously diagnosed with IPNB and similar diseases from each center in Korea, 387 were included in this study after central pathologic review. We also reviewed all preoperative image data. Of 387 patients, 176 (45.5%) had invasive carcinoma and 21 (6.0%) lymph node metastasis. The 5-year overall survival was 80.9% for all patients, 88.8% for IPNB with mucosal dysplasia, and 70.5% for IPNB with invasive carcinoma. According to the “Jang & Kim’s modified anatomical classification,” 265 (68.5%) were intrahepatic, 103 (26.6%) extrahepatic, and 16 (4.1%) diffuse type. Multivariate analysis revealed that tumor invasiveness was a unique predictor for survival analysis. (p = 0.047 [hazard ratio = 2.116, 95% confidence interval 1.010–4.433]). Although IPNB showed good long-term prognosis, relatively aggressive features were also found in invasive carcinoma and extrahepatic/diffuse type. (2) Pathologic subtypes of intraductal papillary neoplasm of bile duct To clarify the clinicopathological characteristics of intraductal papillary neoplasm of the bile duct (IPNB), the Japan Biliary Association (JBA) and the Korean Association of Hepato-Biliary-Pancreatic Surgery (KAHBPS) performed a collaborative study of IPNB based on the new classification. The results of this study is submitted to some journals, so it is hard to fully describe the result before publication. We could show the data at HBP Surgery week after the acceptance of publication. After the central review, we could analyze the characteristics of IPMN by new definition of subtype between Korea-Japan pathologists. Of 694 IPNB patients, 520 and 174 had Type 1 and Type 2, respectively. Type 1 IPNB was more frequently located in the intrahepatic bile duct than Type 2, whereas Type 2 was more frequently located in the distal bile duct than Type 1 IPNB (P<0.001). There were significant differences in 5-year cumulative survival rates and 5-year cumulative disease-free survival rates between the two types. Type 1 and Type 2 IPNBs differ in their clinicopathological features and prognosis.