In pediatric chronic kidney disease (CKD) patients, elevated fibroblast growth factor 23 (FGF23) levels are observed very early in the CKD course (Fig. 1A; 17) and are associated with CKD progression (defined as either dialysis initiation, kidney transplantation, or a 50% reduction in glomerular filtration rate (GFR) from baseline) (Fig. 1B; 57). The association between higher FGF23 levels and CKD progression persists after multivariable adjustment for baseline GFR, proteinuria, blood pressure, glomerular vs. non-glomerular disease, and medication use (angiotensin converting enzyme inhibitors/angiotensin-receptor blockers, vitamin D analogs, and phosphate binders) (57). Used with permission from the American Society of Nephrology.