DTPA renography is commonly used for measuring relative renal function. However, in patients with bilateral renal disease or solitary kidneys, split function studies are of no value. Available techniques to quantify individual renal function are either time consuming or inaccurate. The authors validate the kidney-to-liver ratio at 3 minutes (K3/L3) as an index of renal function, showing excellent correlation with GFR. Normal K3/L3 ranges were then computed in 113 pediatric kidneys (age 2 days to 16 years) and 24 adults. Indicative of renal maturation, the K3/L3 rapidly rose from a value of 1.32 at birth to 2.02 at 6 months. Subsequently, it increased slowly to reach a peak of 2.34 at age 2, followed by gradual decline to adult values after age 5. This decrease is due likely to continued growth of the hepatic blood pool after renal maturation. GFR followed the same maturation pattern to reach a plateau around 2 years of age. K3/L3 reflects individual kidney function, and it requires no blood sampling or urine collection. By establishing normal values at different stages of maturity, this method provides identification and quantification of renal dysfunction in infants, children, and adults.