Osteocalcin, also called bone gla-protein, is a bone matrix protein synthetized specifically by osteoblasts. It circulates in blood where it can be assayed by the radioimmune method. We measured osteocalcin serum levels in 169 adult controls and 161 patients with different disseminated or localized bone diseases. The normal concentration of 6.2 +/- 0.2 ng/ml increases significantly with age. Serum osteocalcin levels are considerably increased in renal osteodystrophy (114 +/- 23 ng/ml) and to a lesser degree in primary hyperparathyroidism (15.9 +/- 2.8 ng/ml) and Paget's disease (11.4 +/- 0.9 ng/ml), all diseases characterized by increased bone turnover. High levels are also encountered in osteomalacia (9.7 +/- 0.9 ng/ml). Conversely, serum osteocalcin levels are significantly decreased in patients under long-term corticosteroid therapy (4.3 +/- 0.5 ng/ml); they remain normal in patients with bone myeloma and bone metastases under treatment. Finally, osteocalcin is normal in patients with osteoporosis, but its level reflects that of bone turnover as evaluated by iliac bone biopsy. The circulating osteocalcin therefore is the first specific and sensitive marker for bone turnover. Serum osteocalcin measurements make it possible to evaluate the osteoblastic bone formation without biopsy and should provide information on the effectiveness of drugs acting on the bone-forming process.