Most patients with inherited hypophosphatemic Rickets/Osteomalacia have mutations in the PHEX gene. In this brief review, we focus on the treatment for patients with this mutation. First, molecular basis of inherited hypophosphatemic Rickets/Osteomalacia, followed by pathophysiology of PHEX and its related disorders is described. Next, clinical manifestation of patients with PHEX mutations and the principles of the treatment are explained. Finally, a case with this mutation that has been long followed up is presented. The most common treatment for this disorder is administration of phosphate and vitamin D, both internationally and in Japan. Degree of the increment in serum inorganic phosphorus levels one hour after phosphate administration, in addition to a decrease in alkaline phosphatase levels is valuable in the monitoring of the treatment. During childhood, markers in a longer term, namely, improvement of X-ray findings and that of height velocity are also useful.