Fibroblast growth factor-23 (FGF-23), a recently identified molecule that is mutated in patients with autosomal dominant hypophosphatemic rickets (ADHR), appears to be involved in the regulation of phosphate homeostasis. Although increased levels of circulating FGF-23 were detected in patients with different phosphate-wasting disorders such as oncogenic osteomalacia (OOM) and X-linked hypophosphatemia (XLH), it is not yet clear whether FGF-23 is directly responsible for the abnormal regulation of mineral ion homeostasis and consequently bone development. To address some of these unresolved questions, we generated a mouse model, in which the entire Fgf-23 gene was replaced with the lacZ gene. Fgf-23 null (Fgf-23^+IhI-/+IhI) mice showed signs of growth retardation by day 17, developed severe hyperphosphatemia with elevated serum 1,25(OH)₂D₃ levels, and died by 13 weeks of age. Hyperphosphatemia in Fgf-23^+IhI-/+IhI mice was accompanied by skeletal abnormalities, as demonstrated by histological, molecular, and various other morphometric analyses. Fgf-23^+IhI-/+IhI mice had increased total-body bone mineral content (BMC) but decreased bone mineral density (BMD) of the limbs. Overall, Fgf-23^+IhI-/+IhI mice exhibited increased mineralization, but also accumulation of unmineralized osteoid leading to marked limb deformities. Moreover, Fgf-23^+IhI-/+IhI mice showed excessive mineralization in soft tissues, including heart and kidney. To further expand our understanding regarding the role of Fgf-23 in phosphate homeostasis and skeletal mineralization, we crossed Fgf-23^+IhI-/+IhI animals with Hyp mice, the murine equivalent of XLH. Interestingly, Hyp males lacking both Fgf-23 alleles were indistinguishable from Fgf-23^+IhI-/+IhI mice, both in terms of serum phosphate levels and skeletal changes, suggesting that Fgf-23 is upstream of the phosphate regulating gene with homologies to endopeptidases on the X chromosome (Phex) and that the increased plasma Fgf-23 levels in Hyp mice (and in XLH patients) may be at least partially responsible for the phosphate imbalance in this disorder.