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Mixed acinar neuroendocrine carcinoma (MAcNEC) of pancreatic type arising in an extrapancreatic location is extremely rare. We present a case of a 70-year-old woman with constipation, abdominal discomfort, and jaundice. Imaging studies revealed a 1.8-cm solid mass in the common bile duct causing dilatation of intrahepatic and extrahepatic bile ducts, which was resected. Microscopically, the tumor was limited to the wall of the bile duct, without involvement of the pancreas, and was composed of solid sheets and nests of relatively monomorphic cells with minimal to moderate amounts of cytoplasm, granular chromatin, focally prominent nucleoli, and up to 160 mitoses per 10 high-power fields. Immunohistochemistry showed equal to or greater than 30% positivity for synaptophysin, chromogranin, chymotrypsin, and trypsin. A diagnosis of MAcNEC was rendered. Immunohistochemistry plays a pivotal role in the identification of these tumors and discrimination from other related entities, neuroendocrine neoplasms in particular. Because pancreatic heterotopia has been described in the common bile duct, we postulate that this case of MAcNEC in the common bile duct, the first to be reported in the literature, may have arisen via malignant transformation of pancreatic heterotopia.