OCT4 is a POU-domain transcription factor that is expressed in embryonic stem cells and germ cells. OCT4 has been detected in specific types of testicular germ cell tumour (GCT), including seminoma and embryonal carcinoma. The aim of this study was to evaluate the role of OCT4 expression in the diagnosis of primary central nervous system (CNS) pure and mixed GCT. Seventeen formalin-fixed, paraffin-embedded tissues of primary CNS GCT were immunohistochemically studied. The 12 pure GCT samples comprised germinoma (5), yolk sac tumour (3), mature teratoma (2), and immature teratoma (2). The five cases of mixed GCT contained various components as follows: yolk sac tumour (4), embryonal carcinoma (3), mature teratoma (1), germinoma (2), polyembryoma (1) and immature teratoma (1). Diffuse and strong nuclear staining indicating OCT4 expression was detected in all cases of pure germinoma (5), and in all cases of mixed GCT containing embryonal carcinoma (3) and/or germinoma (2). There was no corresponding staining in pure GCT of yolk sac tumour, mature teratoma, or immature teratoma except in a primitive neuroectodermal component, or in mixed GCT containing components of yolk sac tumour, mature teratoma or immature teratoma. In conclusion, we found that OCT4 immunostaining is a useful diagnostic tool to assist in the identification of primary CNS embryonal carcinoma and germinoma. In CNS mixed GCT, OCT4 expression can be detected provided that the components include embryonal carcinoma and/or germinoma.