Struma ovarii is a rare form of ovarian neoplasm, composed entirely or predominantly of thyroid tissue and generally a benign germ cell tumor of the ovary. Ascites have been reported in approximately 15-20% of all cases. However, the combination of struma ovarii and elevated CA125 has rarely been reported. We experienced a rare case of a postmenopausal woman with benign struma ovarii associated with massive ascites, a complex pelvic mass. There was marked elevation of her CA125 level. The clinical impression was ovarian malignancy. Surgical excision of the ovarian mass induced immediate resolution of the ascites and a normalization of the serum CA125 level. No recurrence of the ascites or of the tumor has been observed during the 10-month follow-up. Struma ovarii can mimic ovarian malignancy clinically, particularly if complex and associated with ascites and an elevated CA125 level.