The development of bone abnormalities has been studied in 24 patients with severe chronic renal failure. The glomerular filtration rate (GFR) was between 5 and 25 ml/min. The mean values of plasma calcium, degree of bone mineralization (P/Hypro) and bone mineral content (BMC) were subnormal, whereas the mean values of plasma phosphorus and serum parathyroid hormone (PTH) were elevated. Analysis of the data revealed that the various parameters became increasingly pathological with decreasing renal function. Serum PTH correlated inversely with both GFR and plasma calcium. The decrease in bone P/Hypro with decreasing renal function could be explained by an inverse correlation to serum PTH. Plasma alkaline phosphatase correlated inversely to both bone P/Hypro and BMC. The present study on individual patients with varying degrees of renal insufficiency shows that the development of secondary hyperparathyroidism correlates with a reduction in the degree of bone P/Hypro and suggests that significant bone changes appear when the GFR falls below 15 ml/min.