Abstract :Mature cystic teratomas form approximately 20% of all ovarian tumours. Of these, approximately 15% include benign thyroid tissue. When thyroid tissue comprises more than 50% of the ovarian teratoma, it is termed ‘‘struma ovarii’’. The exact incidence of malignancy in struma ovarii is hard to evaluate, because of its uncommon nature. The aim of this study is to report a rare case of follicular variant of papillary thyroid carcinoma arising in mature cystic teratoma of the right ovary. A 43-year-old premenopausal female, gravida 3, para 2 was admitted to Dokuz Eylul University Hospital with an incidental ultrasonographic finding of a complex solid echogenic right ovarian mass during her annual gynecological examination. The preoperative diagnosis was dermoid cyst. The patient underwent laparoscopic surgery. At exploration, the left ovary, the left fallopian tube, and the uterus were normal. The right ovary was enlarged, white and smooth ovarian semisolid mass was noted. Histopathologic evaluation revealed a tumor of follicular variant of papillary thyroid carcinoma arising in a mature cystic teratoma. The case was discussed at the multidisciplinary oncology meeting. Postoperatively, the thyroid function tests, thyroid gland sonogram were requested. The results were normal. A total body scanning with I131 was done. The test revealed no residual intra-abdominal/pelvic carcinoma. Total abdominal hysterectomy, bilateral salpingo-oophorectomy, peritoneal washings, bilateral pelvic-para-aortic lymphadenectomy, partial omentectomy and peritoneal biopsies were performed. In conclusion, the treatment modalities for malignancy in struma ovarii or mature cystic teratoma depend on the stage of the disease and fertility desire.