Studies in the last decade demonstrated that in children tubular maximum phosphate reabsorption per glomerular filtration rate (TmP/GFR) is identical to TP/GFR; TP indicating tubular phosphate reabsorption under basal conditions, without phosphate load. TP/GFR is calculated from the formula TP/GFR=S_P−U_P×S_Cr∶U_Cr, based on simultaneous urine and blood creatinine and phosphate concentrations, and is applicable in both the fasting and non-fasting child. These studies also demonstrated that the use of Walton and Bijvoet nomogram in children may result in overestimation of TmP/GFR compared with TP/GFR calculated from the above formula. When using the formula, one should bear in mind that creatinine is used to express GFR and as a result a significant deviation from true GFR may occur in patients with renal failure. Therefore when employing TP/GFR for the investigation of the renal handling of phosphate in children, three factors should be taken into consideration: (1) the formula in reality expresses TP/C _Cr; (2) only data obtained by exactly the same methodology can be used as reference values; data obtained from studies in which the nomogram was utilized or in which methods other thanC _Cr were used to measure GFR should not be used for reference; (3) in patients with renal failure, TP/C _Cr will significantly overestimate TP/C _inulin.