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We report an unusual case of biliary cystadenocarcinoma with oncocytic differentiation. The patient was a 43-year-old woman who presented with right upper quadrant pain. Imaging revealed a 16 x 10 x 10-cm, heterogenous, right hepatic mass with extension into the right atrium. Surgical resection revealed a papillary neoplasm of malignant cells with atypical hyperchromatic nuclei and prominent nucleoli lining fibrovascular cores. Mesenchymal stroma was not present. The majority of the epithelial cells had abundant eosinophilic granular cytoplasm, consistent with oncocytic differentiation. There was extensive stromal and hepatic parenchymal invasion. Immunohistochemical staining revealed a "biliary pattern" of cytokeratin subset immunoreactivity, with positivity for cytokeratin 7 and an absence of staining with cytokeratin 20. The tumor was negative for mucin, carcinoembryonic antigen, alpha-fetoprotein, calretinin, CD31, and chromogranin. There was granular cytoplasmic staining with phosphotungstic acid hematoxylin, consistent with the presence of abundant mitochondria. Electron microscopy revealed abundant mitochondria within the neoplastic cells. This case is quite unusual because female patients only rarely lack the characteristic ovarian-like mesenchymal stroma of biliary cystadenomas/cystadenocarcinomas. Furthermore, to our knowledge, oncocytic differentiation in this neoplasm has been reported previously on only 2 occasions. The biologic behavior and prognostic significance, if any, of the lack of mesenchymal stroma in female patients or the presence of oncocytic differentiation remains to be further elucidated as more of these cases are described.