This study was undertaken to evaluate the effects of renal transplantation on serum level of free and total PSA. In this study we included 30 male patients with a mean age of 46 years (range 25 to 67) with ESRD undergoing renal transplantation at our department. None of the patients had any history of prostate cancer. All patients had immediate onset of renal function after transplantation, defined by a spontaneous decrease in serum creatinine on postoperative day 1 and a subsequent decrease daily during week 1. Renal transplantation included living related donors in all patients. Blood samples were obtained before and at sixth day after transplantion before removal of Foley catheter. Measurements of fPSA, total PSA were performed with immunofluorometric assays. Glomerular filtration rates were monitored by analyzing serum creatinine. The significance of changes with time was estimated by the Wilcoxon signed ranks test for paired observations with p <0.05 considered statistically significant.: The mean free and total PSA levels before transplantion were 0.22 (range 0.0.05 to 0.4) and 1.5 ng/ml (range 0.1 to 2.9) respectively. There was significant decrease (30% of original levels) in serum fPSA at sixth day after transplantation (p <0.05) in all patients. There was no significant changes at sixth day after transplantation. These results verify the hypothesis that fPSA is eliminated from the blood circulation by glomerular filtration and severe renal failure influences its levels. So we should consider different cutoffs for free to total ratio before and after renal transplantation.