Aims To report the exceedingly rare occurrence of oligodendroglioma in the glial component of ovarian teratomas.

Methods Six cases of oligodendrogliomas arising in the glial component of ovarian teratomas were studied and the literature was reviewed. Immunohistochemistry was performed by the Flex technique.

Results The ages of the patients ranged from 12 to 28 years (mean 21 years). Four tumours were located in the right and one in the left ovary. The size of the ovarian cysts ranged from 7 cm to 29 cm (mean 19.6 cm). Four cases arose in immature and two cases in mature teratomas. In all cases, oligodendroglioma was WHO grade II. On immunohistochemistry, glial fibrillary acidic protein stain was positive in all cases. The Mib 1 (Ki 67) proliferative index was low and the tumour cells were negative for synaptophysin. Follow-up was available in five patients and ranged from 1 to 42 months. Two patients died of disease after 1 and 36 months of diagnosis, respectively. In both these cases oligodendroglioma arose in an immature teratoma. The remaining three patients are alive with a follow-up of 4–42 months.

Conclusions Oligodendroglioma arising in the glial component of ovarian teratomas is exceedingly rare. Ovarian teratomas should be extensively sampled and carefully evaluated to rule out the possibility of a glial tumour. This is the single and largest series of oligodendrogliomas arising in ovarian teratomas. The prognosis is good for oligodendrogliomas arising in mature teratomas compared with those arising in immature teratomas, although long-term follow-up is needed to determine the exact behaviour.