Osteoporosis is a skeletal condition that is characterized by reduction in bone volume and an increased vulnerability to fracture, practically of the proximal femur and vertebrae. Unfortunately, osteoporosis does not usually become obvious until a patient presents with a fracture. For this reason, it has been referred to as the silent epidemic. While radiological examination remains the mainstay of diagnosis, there are certain disadvantages associated with this technique. Almost one third of the skeletal mass must be lost before osteoporosis becomes apparent on a plain radiograph and radiological artifacts are not uncommon. Many methods have been developed for quantitative assessment. of the skeleton. So osteoporosis can be detected early and its progression and response to therapy can be carefully monitored. Recently, quantitative computed tomography(QCT) has been investigated as a means for non-invasive quantitative determination of bone mineral density of the spine. Highly signifcant correlation have been found between vertebral trabecular mineral amount and quantity determined by QCT. Furthermore vertebral strength and failure load have been found to correlate well with trabecular density determined by QCT. Authors has analysed bone mineral density(BMD) of 213 patients who visited Severance Hospital, Yonsei University College of Medicine from Jan. 1988 to Sept. 1989. The aims of this study is to compare BMD of osteoporosis group with that of mormal group, to investigate correlation between BMD and age distribution, and to estimate fracture threshold in osteoporosis group. Following results were obtained:l. After 20 years of age, men lose an average of 0.83 per cent of bone per year, and women lose an average of 0.96 per cent per year. Such lost in women seems to be accelerated during and after menopause. 2. The bone mineral densities for the osteoporosis group are lower than those for the normal group by 28.4% for male and 29.7% for female. There was little difference between sex. 3. It is suggested that 90mg/cm³ of BMD is the fracture threshold for the osteoporosis group. 4. The compression fracture rate among post-menopausal women is 38.8%.