Testicular germ cell tumors (TGCTs) of adolescents and adults have been shown to contain proteins of the human endogenous retrovirus type K family. In a recent study, expression of these retroviral sequences was confirmed using in situ hybridization, which also showed expression in carcinoma in situ, the precursor of all TGCTs. Because of the clinical significance of a test for early diagnosis of TGCTs, we studied whether expression of human endogenous retrovirus type K genes could be an informative parameter. Therefore, we investigated TGCTs of various histologies and testicular parenchyma with and without carcinoma in situ using reverse transcription-polymerase chain reaction for expression of the gag, env, and prt genes. The gag and prt genes were expressed in all samples tested. The env transcripts were not found in TGCTs showing somatic differentiation only but could be detected in most normal testicular parenchyma samples. Therefore, detection of human endogenous retrovirus type K transcripts cannot be used for early diagnosis of TGCTs. Simultaneous expression of multiple gag sequences was found both in normal parenchyma and TGCTs, and we demonstrated that expression of gag sequences with an extra G, necessary to generate a functional protein, was not limited to TGCTs.