A case of carcinosarcoma arising within an otherwise benign cystic teratoma is reported. The patient, a 78 year old nulliparous woman, presented with right sided abdominal pain of short duration and subsequently underwent a bilateral salphingo-oophorectomy. Slicing of the left ovary revealed a unilocular cyst containing hair admixed with soft yellow material with a thin wall apart from a solid area at one pole. Extensive areas of necrosis and cystic degeneration were present within this mass. Histologically, the large cyst was a typical mature cystic teratoma, containing carcinomatous and sarcomatous elements. Mature cystic teratomas have been reported in association with a variety of malignant ovarian tumours such as mucinous cystadenocarcinoma and malignant germ cell neoplasms. Secondary malignant transformation within a dermoid cyst is a much rarer occurrence, estimated as less than 2% of all such lesions. Adenocarcinomas are the second most common malignancies arising within dermoid cysts. Sarcomas alone or in combination with squamous carcinoma have been described arising in a mature cystic teratoma. To the best of our knowledge, no case of sarcoma arising in association with adenocarcinoma has been described before.