A case of small, borderline malignant biliary mucinous cystic tumor is presented. The patient initially presented to us 18 months earlier to undergo a sigmoid colon resection for sigmoid colon cancer. At that time, a liver cyst measuring 18 x 12 mm was detected. On a follow-up abdominal ultrasonography study for colon cancer, the liver cyst had enlarged to 21 mm in diameter and contained a fluid-fluid level 18 months after surgery. Histological examination of a needle biopsy specimen indicated possible adenocarcinoma. Lateral segmentectomy of the liver was performed. Histopathologically, the tumor was diagnosed as a mucinous cystic tumor, of border line malignancy, which had originated from a bile duct gland. It contained both mucinous and serous components, which were thought to have caused the formation of a fluid-fluid level within the cyst. In this case, the fluid-fluid level demonstrated by ultrasonography was beneficial in the early detection of a cystic tumor of the liver. This case may be the smallest reported cystadenocarcinoma of the liver yet published.