Three cases of serous borderline tumors of the ovary with areas of serous low-grade carcinoma metastatic to the anterior mediastinum simulating multilocular thymic cysts are presented. The patients are women between the ages of 33 and 50 years. The 3 women had a prior history of primary ovarian neoplasms diagnosed over a period ranging from 3 to 20 years; the 3 patients were in stages IIIA, IIIB, and III. Follow-up radiologic examination revealed the presence of an anterior mediastinal tumor. The 3 patients underwent surgical resection of the mediastinal tumor. Grossly, the mediastinal tumors measured from 7 to 9 cm in greatest diameter and were described as cystic with solid areas. Focal areas of hemorrhage were present, but frank necrosis was not identified. Histologically, all the tumors basically showed similar histopathologic features, namely, those described in multilocular thymic cysts, ie, cystic structures lined by either squamous or low cuboidal epithelium, lymphoid hyperplasia, cholesterol cleft granulomas, and remnants of thymic tissue. In addition, within the cystic structures, there was a neoplastic cellular proliferation with papillary architecture, nuclear atypia, and scattered mitotic figures. Immunohistochemical studies for keratin, MOC31, and CA-125 showed positive staining in tumor cells while placental-like alkaline phosphatase was negative. Two patients remain alive and well after follow-up ranging from 6 to 18 months and 1 patient died of tumor 18 years after initial diagnosis.