Antibodies against proteins encoded by human endogenous retrovirus (HERV)-K family genes are consistently found in the sera of patients with classical seminoma. Furthermore, HERV-K Gag-encoded protein could be detected in corresponding tumor biopsies. Addressing questions as to the extent of HERV gene expression in biologically related lesions, we studied various testicular and ovarian germ cell tumors (GCTs), GCT precursor lesions, and gestational trophoblastic disease (GTD) for the presence of HERV-K gene transcripts in tissue sections. By in situ hybridization using four non-overlapping, isotopically labeled RNA probes specific for HERV-K gag and env sequences on archival tissue samples, consistent HERV-K expression of gag and env genes was found to be common to all GCTs and their testicular precursor lesions with the exception of teratomas, mature and immature, and spermatocytic seminomas. HERV-K expression was also found in malignant GTD (choriocarcinoma) but not in benign GTD (noninvasive molar pregnancy). There was no evidence for HERV-K expression in differentiated embryonal and adult tissues as well as a total of 53 tumors other than GCT or GTD. The findings point to a common molecular pathoetiology of GCT and malignant GTD, have implications for the classification of GCTs, and support the concept of carcinoma in situ as a precursor lesion common to all forms of testicular GCT.