The renal osteodystrophies represent the metabolic consequences of (1) vitamin D resistance, (2) secondary hyperplasia of the parathyroids, and (3) the changes in serum PO(4)= and Ca++ secondary to the renal insufficiency per se. The osseous lesion in any given patient with chronic renal failure may be osteitis fibrosa, rickets (osteomalacia), calcium deficiency osteoporosis or any combination of these. The concentration of Ca++ and PO(4)= in the serum is determined by the degree of renal failure and the skeletal response to parathyroid hormone.