A rare case of malignant struma ovarii is presented. The tumor was excised together with the ovary and interpreted as a papillary strumal adenocarcinoma arising in a cystic teratoma with predominant thyroid elements. Prior to surgery the patient did not show any symptoms of thyroid hyperfunction. The thyroid function tests performed subsequent to surgery were within normal limits. Periodic examinations of the patient over a period of three years did not reveal any abnormalities. Statistically, this type of malignancy carries a good prognosis with a high "cure" rate.