Nine cases of biliary cystadenocarcinoma of the liver were studied, with emphasis on its clinicopathologic features, mucin profiles, and immunohistochemical characteristics. In general, the cystic tumors had protrusions that consisted of well-differentiated papillary adenocarcinoma cells with or without benign-appearing epithelial elements. In invading or metastatic foci, the carcinoma cells tended to show distinctive anaplastic changes. Tumor growth was confined to the cystic lesions in five cases (noninvasive type), whereas in four cases it extended to the hepatic parenchyma or neighboring organs (invasive type). There was a considerable difference between the two groups in terms of prognosis. In fact, the patients included in the group with the noninvasive type had no sign of tumor recurrence after an appropriate surgical procedure. With mucin histochemical and immunohistochemical approaches, positive reactions with carcinoembryonic antigen, tissue polypeptide antigen, carbohydrate 19-9, and Dupan-2 and the predominance of sialomucin were observed in most cases of biliary cystadenocarcinoma, indicating a similar cellular nature of cholangiocarcinoma.