Struma ovarii has elicited considerable interest because of its many unique features since Ludwig Pick first elucidated its relationship to teratoma in the early part of the 20th century. The most common thyroid-type malignancies to arise in struma ovarii are papillary and follicular carcinomas. In this article, we describe a newly recognized neoplasm originating from struma ovarii that we interpret as follicular carcinoma with a high degree of differentiation. By definition, all of these cases have an innocuous appearance resembling that of nonneoplastic thyroid tissue in both the ovary and sites of dissemination. Including our own, 14 cases in the literature spread to the peritoneum, and 4 metastasized to more distant sites. The peritoneal involvement more often was diagnosed at the same time as the ovarian struma; however, the systemic dissemination occurred subsequent to oophorectomy. Our index patient with highly differentiated follicular carcinoma (HDFCO) developed peritoneal dissemination and para-aortic lymph node metastases 26 years after excision of ovarian struma. Vascular invasion was not identified in any of the cases; however, the primary neoplasm extended to the surface of the ovary in 2 cases with peritoneal involvement. Because of its harmless histological appearance, this form of follicular carcinoma characteristically cannot be diagnosed until the neoplasm spreads beyond the ovary, thus, showing evidence of aggressive behavior. The corollary of this observation is that cases having the histological appearance of ordinary struma ovarii can rarely behave in a malignant fashion. Although cases of typical thyroid-type carcinoma with extraovarian dissemination are relatively easy to diagnose, those with an innocuous histological appearance present nosological and diagnostic difficulties. The differential diagnosis of peritoneal dissemination of struma includes HDFCO, the typical types of thyroid cancer, and so-called strumosis. We have studied the relationship of HDFCO to cases reported as peritoneal strumosis or similar terms and doubt the existence of the latter as a distinct clinicopathologic entity. The treatment of choice for patients with HDFCO is local resection of the extraovarian tumor with subsequent thyroidectomy followed by radioactive iodine ablation.