Familial hypophosphatemia is a hereditary disorder with X-linked dominant inheritance and a high degree of penetrance.1 Some, but not all, affected individuals early in life develop rickets that is highly resistant to vitamin D administration. The most characteristic feature of the disease is phosphaturia, which results from a tubular defect in the parathyroid hormone (PTH)-sensitive component of tubular phosphate reabsorption.2 Untreated hypophosphatemic individuals may have normal PTH concentrations in the blood, but are unresponsive to it.3 The recent demonstration that inorganic phosphate uptake by the intestinal mucosa is also impaired in affected individuals suggests a generalized metabolic defect.4 A unique opportunity to transplant a sister's normal kidney to a severely affected man has provided additional support for this possibility.
This patient was the fifth child of normal parents. His mother was short in stature, but was never known to have bone disease. His father