A single-injection, single-sample procedure for measuring 99mTc-MAG3 clearance is presented that incorporates scaling for patient size and is valid for both children and adults. Methods: The procedure is based on an empirical formula in which all measurements are expressed in dimensionless combinations. The formula was obtained by fitting data collected from 122 adults and 80 children at several centers. Results: All results were scaled to standard adult surface area and are presented in units of ml/min/ 1.73 m2. For adult subjects, the residual standard deviation (r.s.d.) calculated from a single sample at 45 min was found to be 23, using the plasma clearance calculated from a multi-sample clearance curve as a reference. This did not differ significantly from the value of 22 obtained with our previous formula, which was valid for adults only. For pediatric subjects, an r.s.d. of 24 was calculated by the new formula from a single sample at 35 min; a comparable value of 33 was found using a pediatric formula previously published. Conclusion: The new clearance formula is recommended as a replacement for the formula we previously published, since it is based on a larger and more diverse subject population, and since it now holds for children as well, with no loss of accuracy for adult subjects.