Fibroblast growth factor 23 (FGF23) is a circulating protein that regulates the renal reabsorption of phosphate and also inhibits 1-alpha-hydroxylase production. In adults FGF23 is increased in chronic kidney disease (CKD) and is an important prognostic factor for cardiovascular morbidity. In order to gain insight into the role of FGF23 and other biochemical variables of bone metabolism in children we studied 69 patients at different stages of CKD. FGF23 was found to be significantly elevated in stage 3 compared with stages 1 and 2 of CKD, preceding significant hyperphosphatemia in stage 4 disease. The highest levels of FGF23 were found in stage 5 compared with stages 1 and 2 CKD. The levels of FGF23 positively correlated with parathyroid hormone and phosphate concentrations and negatively with 1,25-dihydroxyvitamin D, the estimated glomerular filtration rate, and tubular phosphate reabsorption. Using multivariate analysis, hyperphosphatemia and low estimated glomerular filtration rate remained the most significant factors. Thus we found that FGF23 likely has an important role in pediatric calcium and phosphate homeostasis, and in vitamin D metabolism, even at an early stage of CKD. Further studies are needed to clarify the role of FGF23 on the pathogenesis of renal osteodystrophy and its impact on cardiovascular morbidity in pediatric patients with CKD.