Mature cystic teratoma of the ovary is the most common of the germ cell tumors, representing around 95% of cases. Malignant transformation within a dermoid cyst is a rare event, occurring in 1-2% of all cases and generally affecting postmenopausal women over 50 years of age. In cases of malignant transformation in a teratoma, squamous cell carcinoma is the most common finding (80-90%), followed by adenocarcinoma, carcinoid tumor, sarcoma and
osteosarcoma. Since there are no specific screening tests, the identification of neoplastic cells occurs incidentally on histological examination of surgical specimens. However, in more advanced cases, patients may present with pelvic pain and a palpable abdominal mass at physical examination. Treatment is mainly surgical, involving total hysterectomy with pelvic lymphadenectomy. Chemotherapy can be used in some cases as an adjuvant treatment. Histopathology is critical to confirm diagnosis. This paper reports on a case of malignant transformation within a mature cystic tumor in a woman of reproductive age and describes the clinical and surgical management used.