PURPOSE: It is known that in animal experiments, a very small transplanted kidney relative to the recipient's size caused renal failure by a hyperfiltration injury on the nephron. Currently, studies of post-transplant renal function related to the allograft size are in process at many centers, checking the weight and the volume of the transplanted kidney. The results, however, need to be analyzed because of the diverse outcomes from different analytic points. The proper discriminating point should provide sound predictable factors for obtaining better recipient and graft survival. METHODS: We analyzed the results of renal function according to the different comparative points of the ratios of the kidney weight to the recipient's body weight (KW/BW) and of the kidney volume to the recipient's body surface area (KV/BSA). Among 243 patients who underwent a renal transplant between December 1995 and July 1997, 71 cases were excluded because of early graft loss, acute rejection and incomplete data. The remaining 172 patients were grouped into two groups above and below the various comparative points of the KW/BW and KV/BSA. Thereafter, serum Cr and 24-hr urine protein at 1-yr and 2-yr post- transplantation were compared between the groups. Also we analyzed the correlation between the renal size index (KW/BW, KV/BSA) and the serum Cr, the 24-hr urine protein, and the creatinine clearance at one and two years. RESULTS: From 2.5 to 2.9 of KW/BW, there were relatively consistent results; the transplanted kidneys which had more larger sizes excreted less protein in the 24-hr urine at one year (p<0.05). However, there was no difference in serum Cr level at 1- and 2-yr post-transplantation or in the 24-hr urine protein at 2 yr. Also there were correlations between the KW/BW and the 24-hrurine protein at 1 yr and between the KV/BSA and serum Cr at 1 yr (p<0.05). CONCLUSION: We conclude that the KW/BW, rather than the KV/BSA, was more predictable for post-transplant renal function such as urinary protein excretion, especially at 1 yr. The comparative point, however, should be deliberately adopted by using different multiple point. Also a method of measuring the functional size of the kidney, other than the simple geometric size, needs to be found.