Recent studies have identified a novel bone-kidney endocrine axis that maintains phosphate homeostasis. When phosphate is in excess, fibroblast growth factor-23 (FGF23) is secreted from bone and acts on the kidney to promote phosphate excretion into urine and suppress vitamin D synthesis, thereby inducing negative phosphate balance. One critical feature of FGF23 is that it requires Klotho, a single-pass transmembrane protein expressed in renal tubules, as an obligate coreceptor to bind and activate FGF receptors. Several hereditary disorders that exhibit inappropriately high serum FGF23 levels are associated with phosphate wasting and impaired bone mineralization. In contrast, defects in either FGF23 or Klotho are associated with phosphate retention and a premature-aging syndrome. The aging-like phenotypes in Klotho-deficient or FGF23-deficient mice can be rescued by resolving hyperphosphatemia with dietary or genetic manipulation, suggesting a novel concept that phosphate retention accelerates aging. Phosphate retention is universally observed in patients with chronic kidney disease (CKD) and identified as a potent risk of death in epidemiological studies. Thus, the bone-kidney endocrine axis mediated by FGF23 and Klotho has emerged as a novel target of therapeutic interventions in CKD.