PURPOSE: Struma ovarii is ovarian tumor composed solely or predominantly of thyroid tissue or tumor in which hyperthyroidism results from ovarian thyroid tissue, and usually occurs in tandem with cystic teratoma. Ovarian cystic teratoma is radiologically easily diagnosed due to calcification or fat, for example, but the preoperative diagnosis of struma ovarii is often difficult due to rare characteristic features of thyroid tissue. Our purpose was to determine whether there were specific findings of struma ovarii which distinguished it from other ovarian tumors, and this involved analysis of its radiologic findings. MATERIALS AND METHODS: Using ultrasonography, computed tomography, and magnetic resonance imaging, preoperative radiologic findings of pathologically-proven struma ovarii in eleven patients were retrospectively evaluated for site, margin, nature (cystic, solid, mixed), contrast enhancement, septa, mural nodule, calcification, fat, and metastasis. These findings were compared with pathologic findings. RESULTS: All eleven tumors were unilateral, ten had smooth tumor margins, seven were mixed cystic and solid tumors (more than 70% of solid components in one tumor), and nine had regular septa. Three of four cystic masses (one unilocular, one bilocular and two multilocular cysts) had mural nodules. Calcifications were found in two tumors, and fat in one. Malignant change or metastasis was not found in any tumor. In one patient with hyperthyroidism due to struma ovarii, symptoms and signs of this subsided after removal of the tumor on salpingo-oophorectomy. CONCLUSION: Most cases of struma ovarii occurred unilaterally within ovarian teratomas in premenopausal women, and were mixed cystic and solid masses with smooth margins that are commonly enhanced on contrast enhanced scans. In one patient, hyperthyroidism was caused by struma ovarii. There were, however, no specific radiologic findings that were sufficiently typical to suggest the correct preoperative diagnosis of struma ovarii.