Prompted by the observation of retroviral particle formation in teratocarcinoma cell lines and the consistent finding of antibodies against Gag and Env proteins encoded by human endogenous retrovirus (HERV)-K genomes in the sera of patients with classical seminoma, we studied ovarian and testicular germ cell tumours, their precursor lesions, dysgenetic gonads, and trophoblast lesions for expression of HERV-K sequences by in situ hybridization using radioactive and non-radioactive probes. HERV-K transcripts were detected in all testicular and ovarian germ cell tumours with the exception of teratomas and spermatocytic seminomas. HERV-K expression was also common to testicular carcinoma in situ as well as gonocytes of dysgenetic gonads. Among gestational trophoblastic lesions, HERV-K expression was regularly found in choriocarcinomas, but not in molar lesions. The patterns of HERV-K expression suggest a common molecular pathogenesis of most germ cell tumour entities and malignant gestational trophoblastic disease. They furthermore support the concept of carcinoma in situ as a precursor lesion common to most testicular germ cell neoplasms. The detection of HERV-K gene products in body fluids and tissues may aid diagnosis and monitoring of germ cell tumours and related lesions.