To evaluate renal function quantitatively without blood sampling in children, renal uptake by gamma camera renography using 99mTc-MAG3 was compared with plasma clearance by a single blood sample method as the reference. METHODS Twenty children (15 boys, 5 girls; aged 2-14 yr) with nephrourological diseases were examined prospectively in this study. The patient received an intravenous administration of 5 MBq/kg 99mTc-MAG3 which was prepared using a commercially available kit. Gamma camera renography was performed and the renal uptake per injected dose (%RU) of the 1-min period of postinjection was calculated from a background-corrected renogram curve by computer. The plasma clearance (Clmag) of 99mTc-MAG3 was calculated by the single blood sample method at 35 min postinjection. RESULTS The %RU of the 1-min period in the 1-3 min postinjection correlated well with Clmag. The best correlation was observed 1-2 min postinjection. The regression equation between total %RU (X) and Clmag (Y) (ml/min/1.73 m2) was Y = -98.509 + 20.373X (r = 0.910, s = 84.19) with standardization by BSA. The best fit regression equation between individual %RU (X) and Clmag (Y) (ml/min/1.73m2) was Y = -43.799 + 19.917X (p = 0.932, s = 43.27). CONCLUSIONS The renal uptake method based on separate counts by gamma camera renography using 99mTc-MAG3 does not require a blood sample for quantification of renal function and may be potentially more practical in children.