We reviewed 54 cases of struma ovarii with histologic features diverging from the normal pattern of benign thyroid tissue. These 54 lesions were divided into proliferative struma (41 cases) and malignant struma (13 cases). The patients diagnosed with proliferative struma ovarii ranged in age from 18 to 84 years (average, 44 years). The most common clinical findings among the proliferative struma patients were a mass (58%) and acute abdominal pain (12%). Preoperative evidence of hyperthyroidism was noted in three of the patients with proliferative struma, whereas one additional patient presented with ascites and hydrothorax ("pseudo-Meigs' syndrome"). Proliferative struma differed from the usual struma ovarii in that they comprised areas of densely packed follicles or papillary formations that raised the possibility of malignancy. However, none of the lesions that we have designated as proliferative struma ovarii showed histologic evidence of overlapping "ground glass" nuclei, vascular space invasion, or mitotic activity that would have supported an unequivocal diagnosis of malignancy. None developed metastases or recurrent disease. The 14 malignant struma ovarii manifested the classical features of thyroid carcinoma (including the presence of overlapping "ground glass" nuclei lining papillary formations and vascular space invasion). Patients with malignant struma ovarii ranged in age from 30 to 77 years (average, 50 years). Their clinical presentations included a mass (78%) and acute abdominal pain (22%). One patient had clinical and laboratory evidence of hyperthyroidism. On follow-up, one patient had persistent disease with peritoneal involvement, but distant metastases did not develop in any of these patients. A diagnosis of malignant struma ovarii should be reserved for lesions that exhibit the full range of changes seen in thyroid carcinoma arising in the cervical thyroid. By requiring that these rigid criteria be adhered to, the diagnosis of malignant struma ovarii will probably become less frequent as the more commonly encountered proliferative struma ovarii are recognized.