The limitations of estimates of glomerular filtration rate (GFR) based only on serum creatinine measurements have spurred an interest in more sensitive markers of GFR. Beta-trace protein (BTP), a low-molecular-weight glycoprotein freely filtered through the glomerular basement membrane and with minimal non-renal elimination, may be such a marker. We have recently derived two GFR estimation equations based on BTP. To validate these equations, we measured BTP and the plasma clearance of (99)mTc-DTPA in 92 adult kidney transplant recipients and 54 pediatric patients with impaired kidney function. GFR was estimated using the serum creatinine-based Modification of Diet in Renal Disease (MDRD) Study equation for adults, the Schwartz and updated Schwartz equations in children, and 4 novel BTP-derived equations (our 2 equations and 2 proposed by Poge). In adults, our BTP-based equations had low median bias and high accuracy such that 89-90% of estimates were within 30% of measured GFR. In children, the median bias of our 2 equations was low and accuracy was high such that 78-83% of estimates were within 30% of measured GFR. These results were an improvement compared to the MDRD and Schwartz equations, both of which had high median bias and reduced accuracy. The updated Schwartz equation also performed well.