Background Regulatory T cells (Tregs) are immunologically and clinically interesting not least because of the important role they play in allograft rejection. Likewise, expression of the transcription factor forkhead box protein 3 (FOXP3), detected in transplant biopsies, is also of interest because of its role in the development of regulatory T cells. In this study, we Investigated the relationship between FoxP3 mRNA expression and acute organ rejection in kidney-transplant recipients.Methods In this prospective study, FoxP3 mRNA expression levels in peripheral blood samples from 10 recipients of living relative-donor kidney transplants were measured before transplantation as well as at the 14th and 90th days post-transplantation. In addition, 46 first-time kidney-transplant recipients participated in a cross-sectional study, with 28 patients classified as having acute organ rejection; whilst the remaining 18 patients had functionally stable allografts. FoxP3 mRNA expression levels in peripheral blood samples were compared between these two different groups.Results Before transplantation mean FoxP3 mRNA levels vs. GADPH mRNA levels (lg(FoxP3 mRNA/GADPH mRNA)) in the 10 recipients were 1.11±0.67. The mean FoxP3 mRNA expression levels measured at 14th and 90th days post-transplantation were significantly higher than before transplantation (1.69±0.38, P=0.03; 1.44±0.21, P=0.04, respectively). Additionally, the mean FoxP3 mRNA levels vs. GADPH mRNA expression levels (lg(FoxP3 mRNA/GADPH mRNA)) were significantly higher in recipients suffering acute rejection compared with those with stable allografts (1.77±0.61 and 1.43±0.27, respectively, P=0.03).Conclusions After kidney transplantation, FoxP3 mRNA levels were found to increase in the peripheral blood of all recipients. Considerably higher FoxP3 mRNA levels were observed in recipients suffering acute rejection. These results suggest that FoxP3 mRNA levels in peripheral blood samples can be used as a diagnostic tool for identifying acute rejection.