The contemporary theory for the pathogenesis of X-linked hypophosphatemic rickets/osteomalacia (XLH) has as its central tenet that a primary defect in transepithelial transport of phosphate in the kidney accounts for all of the biochemical and skeletal abnormalities characteristic of the disease. Consistent with this view phosphorus depleted animals develop abnormalities in bone morphology similar to those observed in XLH and treatment of affected subjects with orthophosphate promotes healing of the rickets and linear growth in childhood. Moreover, apparent localization of the proposed defect in renal phosphate wasting to the parathyroid hormone sensitive locus of the proximal renal tubule^1,2 lends further support to the primacy of a renal phosphate leak in the pathogenesis of XLH.