Biochemical indices of bone formation (serum osteocalcin and bone alkaline phosphatase isoenzyme) and osteoclastic function (plasma tartrate resistant acid phosphatase) were measured in 43 patients undergoing chronic hemodialysis and in 27 patients with primary hyperparathyroidism. The mean values for bone alkaline phosphatase isoenzyme and plasma tartrate resistant acid phosphatase but not for osteocalcin were significantly higher in primary hyperparathyroidism as compared with dialyzed patients. A significant positive correlation was found between the biochemical indices of osteoblasts and osteoclasts both in primary hyperparathyroidism and in dialyzed patients, indicating biological coupling between bone resorption and formation under these conditions. The regressions of osteocalcin vs bone alkaline phosphatase isoenzyme and/or plasma tartrate resistant acid phosphatase in dialyzed patients paralleled those in primary hyperparathyroidism but their distance differed significantly. It is concluded that in patients with renal failure, an increase in circulating osteocalcin by a relatively constant portion reflects decreased renal clearance. Any additional increase in osteocalcin serum level indicates an increased skeletal production of osteocalcin. The clinical value of bone alkaline phosphatase isoenzyme and plasma tartrate resistant acid phosphatase appears to be comparable with that of serum osteocalcin in primary hyperparathyroidism, and more exact than osteocalcin in renal failure.