Controversy persists regarding the abnormality of the bone metabolism in various bone diseases. Osteocalcin, the vitamin K-dependent protein synthesized in bone, has been shown to be a specific and sensitive marker of bone turnover in metabolic bone diseases. The author measured the serum osteocalcin level using newly developed radioimmunoassay (RIA) in the patients with osteogenesis imperfecta, metastatic bone tumor, postmenopausal osteoporosis and rheumatoid arthritis, which can be the causes of osteoporosis, and ankylosing spondylitis. The aim of this paper is to demonstrate the value of the osteocalcin measurement comparing the each data of disease group with the normal control group. The results were as followings ; 1. The mean of osteocalcin in normal Korean was 4.43±1.04 ng/ml. (4.45±1.08 ng/ml in male, 4.41±1.01 ng/ml in female). The difference between male and female was not significant statistically. 2. The osteocalcin of osteogenesis imperfects was 9.14±2.21 ng/ml, which was significant statistically, And it was thought to be the result of increasing bone turnover rate and the osteocalcin was useful as biochemical marker. 3. In metastatic bone tumor, osteocalcin was under the normal level in both untreated and treated groups, which meant low bone turnover rate or low osteoblastic activity. Osteocalcin was not a useful marker for the assessment of the effect of treatment in this study, but if the radioimmunoassay was done in the same tumor group, a valuable results could be expected. 4. Osteocalcin level in the patients with postmenopausal osteoporosis and rheumatoid arthritis was not different with normal control group and osteocalcin was not useful marker for the evaluation of the degree of osteoporosis and bone turnover. 5. Osteocalcin didn't play a role in the pathologic ossification of ankylosing spondylitis. In conclusion, the osteocalcin by radioimmunoassay can be used as a biochemical marker in metabolic bone diseases and metastatic cancer for the disgnosis and the assessment of prognosis or the effect of treatment.