BACKGROUND In living kidney transplantation, preoperative donors' renal functional reserve (RFR) may correlate with postoperative residual renal function in both donors and recipients. The aim of this study was to evaluate the donors' RFR before transplantation and to compare basal and stress renal function before and after transplantation in both donors and recipients. METHODS Seven pairs of living kidney donors and recipients were considered for this observational study. RFR was measured with a renal stress test (RST) before and after the kidney transplantation through an oral protein loading test (1 g/kg of body weight). RFR was defined as the difference between the maximum value of creatinine clearance after protein load (stress glomerular filtration rate, sGFR) and baseline creatinine clearance (basal GFR, bGFR). RESULTS Before transplantation, a significant difference between sGFR and bGFR (p = 0.04) was observed in donors, with an RFR = 30.6 (11.9-41.5) mL/min/1.73 m2. After kidney transplantation, sGFR was similar to bGFR for both donors and recipients (p = 0.13), with a limited RFR (7.9 [6.70-19.25] and 14.90 [-6.67 to 25.53] mL/min/1.73 m2, respectively). The sum of the donor's and recipient's post-transplant sGFR was similar to the pre-transplant donor's sGFR (p = 0.73). CONCLUSIONS RST is a safe, feasible, easy, and an inexpensive tool that is able to quantify RFR. In living kidney transplantation, it can be used in clinical practice to measure the original global filtration capacity of the donor's kidneys (sGFR) and to quantify the susceptibility of donors and recipients in developing postoperative kidney dysfunction. However, further studies with an adequate sample size and follow-up period are needed to test this hypothesis.