Clinical Presentation: A 43-year-old female with a history of migraines and hypertension presented with 3 months of dull, constant, right upper quadrant abdominal pain with no associated symptoms. She denied weight loss. Exam was significant for a non-tender abdomen without hepatosplenomegaly, and no stigmata of chronic liver disease. Significant labs were: Cr 0.7, Alb 4, AST 17, ALT 14, AP 64, TB 0.24. A RUQ ultrasound showed a hypoechoic mass appearing in the right lobe of the liver. An abdominal CT revealed a heterogeneous-appearing cyst in the liver with no evidence of ascites. An EGD was performed for evaluation of her abdominal pain, which showed esophagitis. A PPI was started, but she continued to have abdominal pain after several weeks of treatment. An MRI was obtained to further characterize the hepatic cyst, and revealed a 3.9 x 4.3 x 5.1-cm circumscribed cyst within segment 4b, which contained a large enhancing complex mural nodule of 3.2 cm in diameter. The mural nodule was composed of multiple thin and thick septations surrounding tiny cystic spaces. Echinococcus antibodies were weakly positive. She was referred for hepatobiliary surgical consultation. She was given preoperative albendazole for the possibility of an echinococcal cyst. A left hepatectomy and cholecystectomy were successfully performed after several days of albendazole. Surgical findings were significant for a large, mildly steatotic liver with the cyst located predominantly in segment 4b, but also extending into segment 3. Surgical pathology revealed a 5-cm multilocated mucinous biliary cystadenoma, lined by simple cuboidal to columnar mucinous epithelium with ovarian-type stroma, which was negative for malignancy. Immunohistochemical studies showed stromal spindle cells positive for ER and PR. Discussion: Biliary cystadenomas are rare, slow-growing hepatic neoplasms that typically occur in middle-aged women and account for less than 5% of cystic lesions of the liver. The clinical symptoms can be non-specific, but often include abdominal pain. Cystadenomas have the potential for malignant transformation with progression to biliary cystadenocarcinomas, which can be focal and undetectable on imaging. Patients with a suspected cystadenoma should be promptly referred for surgical resection due to the potential for progressing to malignancy, and a high risk of recurrence with non-surgical management strategies.