Purpose To investigate the relationship between ADC values measured by diffusion-weighted MRI (DWI) and the split glomerular filtration rate (GFR). Materials and Methods DWI (b = 0 and 500 seconds/mm²) was performed with a 1.5T MR unit in 55 patients. The ADCs were calculated with ROIs positioned in the renal parenchyma, and the split GFRs were measured by ⁹⁹Tc^m-DTPA scintigraphy using Gates' method. The 110 kidneys were divided into four groups: normal renal function (GFR 40 mL · minute⁻¹), mild renal impairment (40 > GFR ≥ 20 mL · minute⁻¹), moderate renal impairment (20 > GFR ≥ 10 mL · minute⁻¹), and severe renal impairment (GFR < 10 mL · minute⁻¹). The renal ADCs between four groups were statistically compared by analysis of variance (ANOVA), and the relationship between ADCs and GFR was examined using Pearson's correlation test. Results The mean renal ADCs of the four groups were 2.87 ± 0.11, 2.55 ± 0.17, 2.29 ± 0.10, and 2.20 ± 0.11 × 10⁻³mm²/second, respectively. There was a statistically significant difference in renal ADCs among the four groups (P < 0.001). There was a positive correlation between the ADCs and split GFR (r = 0.709). Conclusion The ADCs were significantly lower in impaired kidneys than in normal kidneys, and there was a positive correlation between the ADCs and GFR. J. Magn. Reson. Imaging 2007;26:678–681.