A biochemical and bone biopsy investigation was made in 30 patients with chronic renal failure. Osteomalacia was the main pathological feature in 60% of the patients. Both optical and electron microscopic studies showed that the matrix adjacent to the osteoblasts was not mineralized. Osteomalacia was associated with increased osteoclastic activity and some degree of bone resorption and bone marrow fibrosis in 13% of the patients. In the remaining patients (27%) the bone tissue was substantially normal. The bone calcium content was lower than normal in uraemic patients, whereas a significant increase in magnesium content was observed.The low frequency of the pathological features specific for hyperparathyroidism may be explained by the low-protein, low-phosphorus diets usually followed by the patients. The decrease in bone calcium content depends probably on the reduction in its intestinal absorption and partly on the protein depletion usually observed in patients on low-protein diets. Furthermore, the bone magnesium excess may be regarded as a potentiating factor of bone calcium deficiency.