In living donor kidney transplantation, initial function of the donor kidneys is split into the remnant kidney and the recipient's implanted kidney. We addressed the questions whether the donor's remnant kidney function increases or decreases after donation and whether the donor's donated kidney is greater or less than that of the recipient's implanted kidney after transplantation. We measured and calculated the functional ratio of each kidney using technetium-99m diethylenetriaminepentaacetic acid (99mTcDTPA) as well as serum creatinine (Scr; mg/dL) and creatinine clearance (Ccr; mL/min/1.73 m2) using 24-hour urines from 100 donors. The Ccr was also calculated using the Cockcroft-Gault formula (Ccr-CG; mL/min/1.73 m2). Within 7 days postnephrectomy, we measured Scr, Ccr, and Ccr-CG of the remnant kidney. For recipients, the Scr, Ccr, and Ccr-CG of the implanted kidney with 24-hour urine were obtained within 7 days posttransplantation. The average Scr, Ccr, and Ccr-CG of the donors were 0.85 +/- 0.17 mg/dL, 110.4 +/- 20.8 mL/min/1.73 m2, and 82.8 +/- 17.3 mL/min/1.73 m2, respectively. After donation, the Ccr and Ccr-CG of remnant kidney increased from 54.5 +/- 11.4 and 40.8 +/- 9.3 mL/min/1.73 m2 to 68.0 +/- 14.3 and 53.6 +/- 11.6 mL/min/1.73 m2, respectively. The recipient Ccr and Ccr-CG of donated kidney also increased from 55.9 +/- 11.8 and 42.0 +/- 9.9 mL/min/1.73 m2 to 78.4 +/- 18.0 and 53.4 +/- 16.4 mL/min/1.73 m2, respectively. Kidney transplantation from a living donor should be encouraged with total functional benefit for both donors and recipients.