FGF23 is a hormone that reduces blood phosphate level. Excessive actions of FGF23 result in several kinds of hypophosphatemic rickets/osteomalacia such as X-linked hypophosphatemic rickets and tumor-induced osteomalacia. It is not clear how excessive actions of FGF23 are induced in these diseases. The inhibition of excessive FGF23 actions is a promising new treatment for FGF23-related hypophosphatemic rickets/osteomalacia.