Objective To evaluate the value of renal parenchymal volume and thickness by non-contrast spiral CT in evaluating the differential glomerular filtration rate (GFR) for chronic obstructed kidneys, and to compare the correlations between the two morphologic indices of renal parenchyma and the GFR for chronic obstructed kidneys. Methods Seventy-one patients who had a diagnosis of unilateral chronic upper urinary tract obstruction were included in this analysis. (1) The renal parenchymal volume was mea-sured by non-contrast spiral CT. Both kidneys were scanned by non-contrast spiral CT. The renal parenchymal area of each section was marked manually. Renal parenchymal volume was calculated as the sum of renal parenchymal area multiplied by the width of each section. The volume percentage of obstructed kidney (%CTvol) was also calculated. (2) Renal parenchymal thickness was measured on the first and last non-contrast CT image levels from the anterior, posterior and lateral locations of the kidney that clearly contained the collecting system. The mean of these measurements was defined as the renal parenchymal thickness. The differential renal parenchymal thickness of the obstructed kidney (%CTt) was defined as the percentage of the obstructed renal parenchymal thickness to the total renal parenchymal thickness for both kidneys. GFR was determined with 99Tcm-DTPA dynamic imaging system by Gates method. The differential GFR for obstructed kidney (%GFR) was the GFR percentage of obstructed kidney to the total GFR for both kidneys. The Pearson relation test was carried out between the %CTvol, %CTt and the %GFR respectively. Results %CTvol and %CTt correlated well with %GFR in chronic obstructed kidneys among the 71 test group patients. Pearson correlation coefficient r was 0.80 (t=11.20, P＜0.05) and 0.66 (t=7.24, P＜0.05), respectively. The linear correlation equation respectively was %GFR=0.05+0.80×%CTvol (F=125.48, P＜0.05) and %GFR=0.12+0.66×%CTt (F=52.36, P＜0.05). Conclusions Renal parenchymal volume and thickness by non-contrast spiral CT might be used as clinical practical parameters to evaluate the differential GFR for chronic obstructed kidneys. Renal parenchymal volume is more accurate than renal parenchymal thickness.