Human leukocyte antigen G (HLA-G) is a regulatory molecule that is expressed in the cytotrophoblast during implantation and is thought to allow the tolerance and the development of the semiallogeneic embryo. In vitro, HLA-G inhibits natural killer (NK) cell and CD8 T-cell cytotoxicity. HLA-G also decreases CD4 T-cell expansion. This suggests that it participates in the acceptance of allogeneic organ transplants in humans. We here describe the detection of high concentration of HLA-G in serum from liver-kidney transplant patients, but not in kidney transplant patients. This finding is supported by the ectopic expression of HLA-G in graft biopsies. Finally, its association with a low number of acute transplant rejections, especially in liver-kidney transplant patients led us to propose that HLA-G may serve to monitor transplant patients who are likely to accept their allograft and, thus, may benefit of a reduced immunosuppressive treatment.