Histomorphometry was performed on transiliac bone biopsies, double-labeled with tetracycline, from 60 consecutively admitted patients (20 women) at various stages of chronic renal failure (CRF). Eleven patients (1 woman) had normal bone resorption and formation indices. Bone resorption and osteoid formation increased with progression of renal failure, but abnormal values were seen even at slightly elevated creatinine levels. Mineralization lag time increased with CRF duration; prolonged values were only seen in patients with polycystic kidney disease or chronic pyelonephritis with advanced CRF. All patients with impaired mineralization also had increased bone resorption. Diabetes mellitus did not protect against skeletal lesions. The biochemical tests were too insensitive to predict type or severity of bone disease, and hand X-rays had no diagnostic value in early stages of renal osteodystrophy.