Extrahepatic bile duct cancer with an endocrine cell component has rarely been reported. We report here on a case of adenoendocrine cell carcinoma in the middle bile duct. An 82-year-old man was admitted to hospital for jaundice and anorexia. Computed tomography and magnetic resonance imaging examination showed a papillary low-density mass in the middle bile duct. Endoscopic retrograde cholangiography showed obstruction of the bile duct, and blushing cytology of the bile duct revealed an adenocarcinoma. We resected the extrahepatic bile duct with regional lymph node dissection. A pathological examination revealed a neuroendocrine component showing small cytoplasmic cells with hyperchromatic nuclei and a rosette-like structure in the middle of the tumor. In the peripheral mucosal region, there was a well-differentiated adenocarcinoma composed of columnar and cuboidal epithelial cells with clear and slightly granular eosinophilic cytoplasm. Immunohistochemical analysis showed positive staining for CD56, following the diagnosis of adenoendocrine cell carcinoma. The Ki-67 rate was &gt
30% suggesting a small-cell endocrine carcinoma. The adenocarcinoma component infiltrated into the endocrine component, and some of the endocrine component was positive for cytokeratin, suggesting transdifferentiation of the adenocarcinoma into the endocrine component rather than originating from the common precursor cell. The patient experienced liver metastasis 3 months after the operation and died 6 months after the operation. Adenoendocrine tumor of the bile duct is extremely rare and adjuvant chemotherapy is necessary according to the malignant potential of the neuroendocrine tumor rather than the adenocarcinoma. © 2011 Springer.