Objective　To investigate the gene expression of 4-1BB in peripheral blood mononuclear cells (PBMCs) and the possible significance of the 4-1BB pathway after clinical orthotopic liver transplantation (OLT). Methods　4-1BB mRNA levels in PBMCs from 22 OLT patients were analyzed by RT-PCR. 4-1BB protein expressed on the surface of CD4+ and CD8+ T cells were detected by flow cytometry, and visualized with direct immunofluorescence and confocal fluorescence microscopy. Patients with primary liver cancer (PLC) and healthy volunteers served as controls. Six cases of recently performed liver transplantation were also observed in this study.Results　4-1BB mRNA was detected in PBMCs from both liver transplant patients with long-term graft acceptance (22 cases) and from transplant patients on day 1 to day 3 post-transplantation (6 cases), but was not found in PBMCs from transplant patients on day 7 to day 30 post-transplantation (6 cases). 4-1BB mRNA was also not found in samples from 8 of the healthy controls and 7 of the PLC patients, though very low expression was detected in the other 4 healthy volunteers and 6 PLC patients. Simultaneously, 4-1BB protein was expressed at nearly undetectable levels on CD4+ and CD8+ T cells from healthy controls, PLC patients, as well as OLT patients within the first month post-transplantation (6 cases). However, 4-1BB expression was found on the surface of CD4+ and CD8+ T cells from liver transplant patients with long-term graft acceptance. Direct immunofluorescent staining and confocal fluorescence microscopy clearly revealed evidence of 4-1BB protein on cell membranes of CD4+ and CD8+ T cells from liver transplant patients with long-term graft acceptance. Simultaneously, a significantly higher percentage of CD3+ CD25+ T cells were found in liver transplant patients with long-term graft acceptance group as compared with the healthy control group (P<0.05). The expression of 4-1BB protein on T cells did not correlate with the survival time of OLT patients postoperation.Conclusions　This study demonstrates that although patients remain in stable condition after liver transplantation under the treatment of immunosuppressants, activated T cells are present to some extent and 4-1BB protein may be involved in this process. Effector T-cells can exert permanent immunoresponses against grafts under these circumstances. Therefore, we conclude that a new immune response balance is established under the combination of both treatment with immunosuppressants and natural immune responses against alloantigens. Manipulation of the 4-1BB/4-1BBL pathway may provide a therapeutic technique for prolonging graft survival.