The formula GFR = kL/Pcr can be used to estimate GFR in infants, children, and adolescents who have grossly normal body habitus and are in steady-state condition. GFR is expressed in ml/min per 1.73 m2 BSA, L represents body length in cm, Pcr represents plasma creatinine concentration in mg per dl and k is a constant of proportionality that reflects the relationship between urinary creatinine excretion and units of body size. The value of k varies as a function of age and sex being 0.33 in preterm infants, 0.45 in full-term infants, 0.55 in children and adolescent girls, and 0.70 in adolescent boys. The advantages of rapid determination, reasonable accuracy, and the avoidance of urine collection justify the use of this formula in pediatric patients.