Fibroblast growth factor (FGF) 23 is a humoral factor that reduces serum phosphate and 1,25-dihydroxyvitamin D [1,25 (OH)(2)D] levels at least in part by suppressing proximal tubular phosphate reabsorption and 1,25 (OH)(2)D production. It has been shown that several hypophosphatemic diseases characterized by impaired phosphate reabsorption, relatively low 1,25 (OH)(2)D and rickets/osteomalacia are caused by excess actions of FGF23. Measurement of circulatory FGF23 levels seems to be useful for diagnosis of these hypophosphatemic diseases. Novel therapeutic methods for these diseases may be developed by finding maneuvers to modulate FGF23 actions.