Struma ovarii is the most common monodermal ovarian teratoma and consists mainly of thyroid tissue. Only 5% of patients with this tumor have features of hyperthyroidism. The pathophysiology of hyperthyroidism in struma ovarii is not clear.We describe a case of benign struma ovarii, presenting with the clinical features of an ovarian cancer: large complex pelvic mass, large amount of ascites and markedly elevated CA-125 serum levels. The patient was initially treated for Graves' disease, on the basis of ultrasonographic, laboratoristic and scintigraphic evidence. The resistance to the medical treatment led to thyroidectomy. After surgery the hyperthyroidism persisted and, suddenly, the patient presented ascites. A large pelvic mass was then diagnosed which, at the pathologic examination, was diagnosed as a struma ovarii.The struma ovarii always has to be considered when a pelvic mass is associated with features of hyperthyroidism.