We present the first case of an intraductal papillary neoplasm of the bile duct(IPNB) accompanying a mixed adenoneuroendocrine carcinoma(MANEC).A 74-yearold woman presented with fever of unknown cause.Laboratory data revealed jaundice and liver injury.Contrast-enhanced computed tomography revealed a 20 mm polypoid tumor in the dilated distal bile duct,which exhibited early enhancement and papillary growth.Upper gastrointestinal endoscopy revealed mucus production from the papilla of Vater,characterized by its protruding and dilated orifice.Endoscopic ultrasonography visualized the polypoid tumor in the distal bile duct,but no invasive region was suggested by diagnostic imaging.Therefore,the initial diagnosis was IPNB.After endoscopic nasobiliary drainage,a pylorus-preserving pancreaticoduodenectomy was performed.Pathological examination of the resected bile duct revealed papillary proliferation of biliary-type cells with nuclear atypia,indicating pancreaticobiliary-type IPNB.In addition,solid portions comprised of tumor cells with characteristic salt-and-pepper nuclei were evident.Immunohistochemistry revealed expression of the neuroendocrine marker synaptophysin in this solid component,diagnosing it as a neuroendocrine tumor(NET).Furthermore,the MIB-1 proliferation index of NET was higher than that of IPNB,and microinvasion of the NET component was found,indicating neuroendocrine carcinoma(NET G3).This unique case of MANEC,comprising IPNB and NET,provides insight into the pathogenesis of biliary NET. We present the first case of an intraductal papillary neoplasm of the bile duct (IPNB) accompanying a mixed adenoneuroendocrine carcinoma (MANEC). A 74-yearold woman presented with fever of unknown cause. Laboratory data revealed jaundice and liver injury. Contrast-enhanced exam tomography revealed a 20 mm polypoid tumor in the dilated distal bile duct, which exhibits early enhancement and papillary growth. Upper gastrointestinal endoscopy revealed mucus production from the papilla of Vater, characterized by its protruding and dilated orifice. Endoscopic ultrasonography visualized the polypoid tumor in the distal bile duct, but no invasive region was suggested by diagnostic imaging. Before, the initial diagnosis was IPNB. After endoscopic nasobiliary drainage, a pylorus-preserving pancreaticoduodenectomy was performed. Pathological examination of the resected bile duct revealed papillary proliferation of biliary-type cells with nuclear atypia, suggesting pancreaticobiliary-type IPNB. additio n, solid portions comprised of tumor cells with characteristic salt-and-pepper nuclei were evident. Immunohistochemistry revealed expression of the neuroendocrine marker synaptophysin in this solid component, diagnosed it as a neuroendocrine tumor (NET). Thermal Co., MIB- of NET was higher than that of IPNB, and microinvasion of the NET component was found, indicating neuroendocrine carcinoma (NET G3). This unique case of MANEC, including IPNB and NET, provides insight into the pathogenesis of biliary NET.