The identification of FGF23 as a factor involved in several disorders of phosphate regulation and of PHEX as the gene mutated in X-linked Hypophosphatemic Rickets indicates that both these genes may be involved in phosphate homeostasis, although their physiological roles are unclear. In this study, FGF23 mRNA expression was analyzed by real-time RT-PCR and found to be higher in normal human bone than in kidney, liver, thyroid, or parathyroid tissue, while expression in oncogenic osteomalacia tumor tissue was several hundred-fold higher than in bone. Expression of FGF23 mRNA in human osteoblast-like bone cells, quantitated by real-time RT-PCR, increased with increasing extracellular phosphate and was 2-fold higher in cells treated with 2 mM extracellular phosphate compared to 0 mM phosphate treatment. PHEX mRNA expression increased 1.3-fold after treatment with 2 mM phosphate. FGF23 expression in the bone cells increased with increased mineralization over a 20-day treatment period under mineralizing conditions with beta-glycerophosphate, while PHEX expression decreased. The results indicate that FGF23 mRNA expression in bone cells is regulated by extracellular phosphate and by mineralization. These results support proposals that bone may be a source of circulating FGF23 and suggest that FGF23 expression by bone is regulated.