Scince the introduction of irradiated ergosterol into our food supply, nutritional vitamin D-deficiency rickets has become an uncommon disease. However, skeletal disorders due to abnormalities of vitamin D function still occur. These disorders can now be classified more exactly into two groups: those in which there is a deficiency of the active metabolite of vitamin D, 1,25-dihydroxyvitamin D, and those in which there is an abnormality of renal tubular function resulting in renal hypophosphatemia despite normal vitamin D metabolism. The various entities of these two groups are described and the theoretical basis of their treatment given.