By using a 125I collimated point source to measure bone mineral, 700 measurements were obtained from the cortical bone of the lower arm in 303 patients with renal failure representing every stage of the disease. Of these patients, 123 had received transplants. The following conclusions can be drawn: (a) Prolonged azotemia leads to severe loss of bone mineral, which, in early stages, is only detectable using photon absorptiometry. (b) Gradual loss of bone mineral occurs during dialysis, (c) Parathyroidectomy slows this loss but fails to reverse it. (d) After transplantation, the rate of bone loss will either decrease or cease, but few patients show any increase in bone mineral. (e) Preliminary biochemical data on these patients show a relationship between bone mineral and creatinine, serum phosphate and serum calcium values.