Fibroblast growth factor (FGF) 23 has been identified as the last member of FGF family. FGF23 reduces serum phosphate level by suppressing proximal tubular phosphate reabsorption and intestinal phosphate absorption. FGF23 is produced by bone and acts on the kidney through a specific receptor system which is composed of Klotho and certain subtypes of FGF receptors. Excess actions of FGF23 cause several hypophosphatemic diseases characterized by impaired renal phosphate reabsorption and rickets/osteomalacia. In contrast, deficient actions of FGF23 result in hyperphosphatemic tumoral calcinosis with enhanced renal phosphate reabsorption. These results indicate that FGF23 works as a hormone to regulate the serum phosphate level.