To survey and characterize intraductal polypoid neoplasms in the intrahepatic large bile ducts of small duct-type intrahepatic cholangiocarcinoma (small duct-iCCA), a total of 121 cases of small duct-iCCA presenting mass-forming growth were surveyed for intraductal polypoid neoplasms that were compared with mass-forming tumors in individual cases and with intraductal papillary neoplasm of bile duct (IPNB) (20 cases). Polypoid neoplasms were found in intrahepatic bile ducts in 8 (6.6%) of 121 cases of small duct-iCCA. They showed cast-like growth involving several adjoining bile ducts adjacent to or in the peripheries of mass-forming tumors as well as well-differentiated papillary or tubular/cribriform patterns and no stromal invasion. Intraductal polypoid neoplasms were histologically and immunohistochemically similar to mass-forming tumors in individual cases, and both components were of biliary subtype. There was an abrupt transition between these polypoid neoplasms and normal lining epithelia in the affected bile ducts, suggesting that intraductal polypoid neoplasms reflect the cancerization of ducts. IPNB presented with biliary (5 cases), intestinal (8 cases), gastric (5 cases), and oncocytic subtypes (2 cases), and about half of IPNBs were noninvasive, thus differing from intraductal polypoid neoplasms of small duct-iCCA. In conclusion, small duct-iCCA occasionally presents as intraductal polypoid neoplasms in adjoining bile ducts, reflecting the cancerization of ducts. These intraductal polypoid neoplasms should be considered in the differential diagnosis of heterogeneous intraductal tumors of bile ducts.