The glomerular filtration rate (GFR) may increase after oral protein overload or after an amino acid infusion. Renal functional reserve (RFR) is defined as the difference between the poststimulation and the baseline glomerular filtration rate. RFR has been studied in transplant patients. The results have been conflicting because RFR may be affected by donor age, the time from transplantation, the type of immunosuppression, the basal GFR, the body surface area, and the presence of calcium channel blocker therapy. We observed that during the first year posttransplantation renal recipients maintained on cyclosporine (CsA) therapy retain a level of RFR that represents approximately 30% of the baseline GFR. Moreover, hypertensive cyclosporine treated renal transplant patients on calcium channel blocker therapy do not exhibit permanent glomerular hyperfiltration until 8 months posttransplantation. Finally, both renal and heart transplant patients on cyclosporine therapy possess renal reserve, but the single renal graft in renal transplant patients shows a proportionally higher baseline GFR and a better ability to exhibit a RFR than the two native kidneys in heart transplant patients. Many studies, although not all, have documented a positive RFR in both children and adult cyclosporine-treated kidney graft recipients, demonstrating that hyperfiltration is not the rule following single kidney transplantation.