Gonadoblastomas are rare tumours of abnormal or dysgenetic gonads, often transforming to invasive seminomatous and nonseminomatous germ cell tumours (GCT). Because of the intimate association of noninvasive and invasive lesions, gonadoblastoma may provide clues as to the molecular pathogenesis of GCT. We studied the expression of the human endogenous retrovirus (HERV)-K gag gene in eight gonadoblastomas arising in phenotypically female patients, including two newborn girls. We also studied testicular biopsies with immature Sertoli cell nodules harbouring neoplastic germ cells, a lesion with morphological resemblance to gonadoblastoma. In five gonadoblastomas, invasive seminoma/dysgerminoma was noted, in two cases with formation of additional GCT components. HERV-K gag transcripts were found with moderate levels in gonocytes of all gonadoblastomas and in neoplastic germ cells in testicular Sertoli cell nodules. All invasive GCT except for teratomas displayed HERV-K transcripts. Thus, expression of HERV-K is induced during fetal or embryonal development and precedes invasive GCT formation. Although the specific role of HERV-K expression remains unknown, the findings place HERV-K expression in an appropriate time frame for it to have a role in the molecular pathogenesis of GCT and suggest a precursor-invasive tumour relationship for ovarian GCT equivalent to the more common carcinoma in situ of the testis and testicular GCT.