Biliary Cystadenocarcinoma (BCC) is characterized histologically by a dense mesenchymal stroma covered by malignant epithelium. Illustrated here is the mesechmyal stoma with a benign epithelial layer. Sometimes lesions will have variable epithelium, some benign and some malignant. If any malignant epithelium is seen, the lesion is classified as malignant. Interestingly, the nature of the stroma, not the epithelium, is related to prognosis. Those with the stroma have a better prognosis. This lesion is identical to the mucinous tumor of the ovary, the mucinous cystic tumor of the pancreas, and the biliary cystadenoma.