Bone is a dynamic tissue that functions not only as a mechanical support, but also as a major component of the metabolic and endocrine systems maintaining mineral homeostasis. It has been shown that immobilization induces decalcification of bone. To evaluate the effect of immobilization on bone mineral density and calcium metabolism, we investigated 9 young healthy males and females during 20 days bed rest. Three methods for measuring bone mineral density were performed to quantify whole body and regional bone changes: 1) dual energy X-ray absorptiometry, 2) quantitative computed tomography, and 3) multiple scanning X-ray photodensitometry, respectively. Bone mineral density showed a rapid decreasing tendency, especially in both lumbar and metacarpal bones (mean +/- SE: 4.6 +/- 0.6% and 3.6 +/- 0.4%, respectively). Urinary daily excretion of deoxypyridinoline, a sensitive marker of bone matrix resorption, tended to increase by day 10, and to decline by day 20 (mean +/- SE: 42.2 +/- 1.4, 27.6 +/- 2.2 nmol day-1, respectively). However, neither alkaline phosphatase nor tartrate-resistant acid phosphatase, both markers of osteoclast and mature osteoblast function, changed. These results showed that in the early stage of immobilization, bone matrix might be resorbed without any activation of osteoclasts, resulting in rapid decalcification of vertebral and cortical bones without any discernible changes in anatomical structure.