Human endogenous retroviral sequences (HERVs) are believed to be possible pathogenic agents in carcinogenesis. HERV-K is the most biologically active form, since members of this family have intact open reading frames for the gag, pol or env genes. Antibody response against HERV-K peptides has been reported in leukemia patients, suggesting a possible overexpression of this sequence in leukemic cells. Using real-time quantitative RT-PCR (TaqMan), we found that in six of the eight leukemia samples we collected, transcriptional activity of HERV-K10-like gag gene was 5- to 10-fold higher than in normal peripheral blood mononuclear cells (PBMCs) or mononuclear cells from cord blood. The overexpression was marked enough to be detected by Northern blot. In addition, there was no significant variation of HERV-K expression in normal PBMCs after exposure to different factors (PHA, gamma irradiation, 5-azacytidine) that potentially modulate HERV expression. This suggests that HERV-K relative overexpression in leukemia samples might be specifically associated with tumor development. The origin of these transcriptional variations is therefore worth being investigated further.