The discovery of the vitamin D endocrine system has opened up many possibilities in our understanding of metabolic bone disease. Of particular importance is the fact that we can now manage certain genetic disorders resulting in vitamin D-resistant rickets or vitamin D-resistant hypocalcemia with the new active hormonal forms of vitamin D and with intelligent dietary management to provide for correction of the mineral difficulty. Thus, in the case of vitamin D dependency, replacement need only be with the missing hormone, 1,25-(OH)2D3. On the other hand, familial hypophosphatemia requires adjustment of the plasma phosphorus by frequent administration of oral phosphate and the adjustment of intestinal calcium absorption by 1,25-(OH)2D3. Renal failure patients require the adjustment of plasma phosphorus concentration and parathyroid hormone status, and the administration of the missing hormone 1,25-(OH)2D3. Hypoparathyroid patients require oral calcium plus 1,25-(OH)2D3, and premature infants require administration of the 1,25-(OH)2D3 because the immature kidneys and immature parathyroid glands fail to produce the required amount of this hormone. Other vitamin D-resistant rachitic conditions cannot be discussed here for lack of space and for lack of information. Undoubtedly, such patients as those having rickets secondary to renal tubular acidosis and rickets secondary to hepatic disorders will eventually come under effecti dietary and hormonal management. In this sense, the vitamin D endocrine system and vitamin D-resistant rickets can serve as a prototype of management of a genetic disorder by dietary means.