BACKGROUND: Biliary cystadenoma is a very rare cystic neoplasm of the liver. Its clinical features, diagnosis, pathologic characteristics, and optimal surgical management have not been deifned clearly. In this article we describe the details of this rare disease. METHODS:A 40-year-old woman with a mass of the liver was veriifed by ultrasonography and LT. Ultrasonography showed a mixed echo of 18.4 cm×14.72 cm×15.54 cm in the left lobe of the liver. CT showed a vesicula of 19.9 cm× 13.5 cm in the right epigastrium, with a low density, clear edge, uneven density, and calcified shadow. The patient received successfully a left hepatectomy. Laboratory examination showed an elevation of CA125 to 62.62 U/ml and CA199>1000 U/ml. RESULTS:After the left hepatectomy, the patient was fully recovered. Her biliary cystadenoma was characterized by speciifc histological ifndings. During operation, a large cystic lesion was seen in the left hepatic lobe; its surface was dark red with abundant blood supply. Gross examination showed that the tumor almost occupied. The whole left lobe with a small amount of normal liver tissue close to the deltoid ligament. Pathologically, additional lobulated spaces were seen in the tumor with a lot of mucusa. The interior wall was lined with bile duct tissue, indicating a benign mucinous biliary cystadenoma. CONCLUSIONS: Ultrasonography and CT are the major methods for the diagnosis of mucinous biliary cystadenoma liver. Operation is the best way of treatment.