A case of mucinous cystadenocarcinoma arising from a mature cystic teratoma in the right ovary of a 37-year-old woman is reported. Malignant transformation of a mature teratoma is a rare event and the commonest malignant neoplasm to develop is squamous cell carcinoma.(83%) Adenocarcinoma occurs with less frequency.(6.8%) The patient was treated by staging operation followed by adjuvant chemotherapy. After six courses of combination chemotherapy, oncological investigations, including chest X-ray, abdominopelvic CT scan and tumor markers, all revealed no evidence of recurrence.