BACKGROUND: Previous findings demonstrated that the expression of cytotoxic effector molecules is increased in acute rejection of renal allografts. In the present study, we serially examined the gene expression of perforin, granzyme B and Fas ligand(FasL) in peripheral blood lymphocytes(PBLs) of renal allograft recipients to assess the potential of their expression as a marker of acute rejection. METHODS: PBLs were isolated from blood samples taken on days 2, 4, 6, 8, 10 and 12 after transplantation. Competitive PCR was performed to evaluate the abundance of mRNA of perforin, granzyme B and FasL. The mean value of each molecule plus 2 SD for the control group was set as a discriminatory level. RESULTS: When all measured samples were compared, perforin expression was significantly higher in patients with acute rejection than in the control group(1.84+/-3.01 versus 0.71+/-0.48, p=0.01). The percentage of perforin expression exceeding the discriminatory level was also significantly higher in patients with acute rejection(p=0.0003). Five patients in the rejection group(5/7, 71.4%) showed perforin expression exceeding the discriminatory level, while only 1 patient in the control group did so(1/8, 12.5%)(p= 0.02). Perforin expression of days 0 and 1 of rejection crisis was the highest over the study period. No consistent pattern of granzyme B and FasL expression was identified in relation to rejection crisis. CONCLUSION: Gene expression of perforin by PBLs was upregulated in accordance with acute rejection, thus offering the possibility that it may be utilized as a marker of acute rejection.