Ovarian monodermal teratoma containing thyroid tissue is a rare tumor. Authors reviewed the records of 94 patients operated for ovarian teratoma in their department between 1986 and 2002, of which 3 patients proved to have struma ovarii. They found that the 3 patients had highly variable clinical symptoms and morphological characteristics including tumor size, pain as main symptom, and the presence or absence of ascites and hormonal activity. It is concluded that struma ovarii appears to be poorly diagnosed, but an extensive histological examination may increase the diagnostic efficacy.