PURPOSE: To prospectively evaluate feasibility of diffusion-weighted (DW) magnetic resonance (MR) imaging in assessment of renal function in healthy volunteers and patients with various renal abnormalities and to prospectively evaluate reproducibility of DW MR imaging in volunteers. MATERIALS AND METHODS: Study protocol was approved by local ethics committee; informed consent was obtained. Eighteen healthy volunteers and 15 patients underwent transverse fat-saturated echo-planar DW MR imaging of the kidneys during normal breathing. Freehand regions of interest were delineated in the cortex and medulla of the kidneys. The following apparent diffusion coefficient (ADC) values were calculated: ADC of all b values (ADC(avg)), ADC of low b values (b = 0, 50, 100 sec/mm2; ADC(low)), and ADC of high b values (b = 500, 750, 1000 sec/mm2; ADC(high)). These values were calculated to differentiate influence of perfusion and diffusion. Reproducibility was assessed by repeating the same protocol in five randomly selected volunteers after 6 months. For statistical analysis, Student t tests were used. RESULTS: In all volunteers, ADC(avg) and ADC(high) were significantly higher in the cortex than in the medulla (P < .001). No difference between the cortex and medulla could be observed for ADC(low). Patients with renal failure had significantly lower ADC(avg) (P < .001, P = .004), ADC(low) (P = .02, P = .03), and ADC(high) (P = .02, P = .04) of cortex and medulla, respectively, than did volunteers. In the patient with pyelonephritis, all ADC values of cortex and medulla were substantially lower compared with the contralateral side, whereas patients with ureteral obstruction showed varying degrees of difference in all ADC values compared with the contralateral side. No statistically significant changes were found in the repeat study of the volunteers. CONCLUSION: DW MR imaging is feasible and reproducible in the assessment of renal function, as shown in our initial experience with a small number of patients and volunteers.