Background: Relative kidney function is an important parameter during urological decision making. Objective: In this study, we evaluated the feasibility of estimating relative kidney function from relative kidney parenchymal volume determined by non-enhanced CT. Design Setting and Participants: Between the years 2005 to 2007 twenty-seven patients underwent both non-enhanced CT and renal scintigraphy within three months in our hospital. The indications for CT scanning were: evaluation of stone disease in 18 patients (66%), workup before or after a urological procedure in 7 (26%), suspected ureteral obstruction and prostate cancer with urosepsis each in a single patient. Intervention: A region of interest (ROI) was manually drawn around the edges of each kidney, excluding the collecting system, renal cysts and tumors. Kidney volumes were extrapolated by summation of all measured ROIs on a dedicated workstation. Relative kidney volume was calculated as the ratio between individual and total kidney volumes. Measurements: Relative kidney volume as measured by CT was compared to relative kidney function as measured by scintigraphy. Results and Limitations: Relative kidney volume correlated well with relative kidney function (Pearson correlation coefficient r = 0.92), and this good correlation prevailed over a wide range of relative kidney functions (from 15% to 87%). Mean individual difference between the CT and scintigraphy ratios was 5.7% (S.D. 4.6%). In 89% of the patients the difference was less then 10%, and in none more than 18%. Kidney morphological configurations on non-enhanced CT closely resembled the functional configuration on scintigraphy. This preliminary study is limited by the small number of cases and cases of acute deterioration in kidney function were not included. Conclusions: In this preliminary study we showed that relative kidney parenchymal volume measured by nonenhanced CT images may provide a close estimate of kidney relative function.