No new general review has appeared concerning the malignant changes that can occur in dermoid cysts of the ovary since the works of Peterson in 1957 and Climie and Heath in 1968. A personal case of epidermoid carcinoma--Stage II B--which occurred in a dermoid cyst gave us the opportunity of reviewing the 132 cases published between 1968 and 1985, which bring the total cases published to more than 400. The cancer arises in 1.78% of mature cystic teratomas. The mean age at diagnosis is 53 years and this pathological change is found five times more frequently after than before the menopause. Before laparotomy is carried out there is no clinical sign nor paraclinical evidence of the diagnosis over and above that of a benign cyst. There are all kinds of histological changes but the epidermoid carcinomas occur in 75% of cases. The survival rate for this kind of ovarian tumour is very poor, with 27% survival after 3 years, taking into account all states and all histological types. Epidermoid carcinomas seem to have a better prognosis than other histological types, and an important prognostic feature is the spread and the ability to remove the tumour surgically when it has extended beyond the capsule of the ovary. Although it is not easy to lay down the best therapy for this condition, by and large extended surgery followed by polychemotherapy or radiotherapy are apparently the only ways of improving the poor prognosis for these tumours.