The aims of the treatment of osteoporosis are to halt the rapid decrease in bone mass, and if possible, to increase the bone mass. Appropriate treatment is expected to lower the incidence of bone fracture in patients with a low bone mass by maintaining or increasing bone mass on a long-term basis. In our experience [1], cases of senile osteoporosis with low bone mass showed a significantly lower incidence of vertebral fracture after 2 years of treatment with lc~-hydroxyvitamin D 3 (I~z-OHD3). This observation suggests that sustained bone mass is effective in preventing further fracture. However, there has been some criticism of active vitamin D 3 administration. Namely, active vitamin D 3 treatment of osteoporosis has been reported to be both effective [2-7] and ineffective [8-10]. Thus the aims of the present study were to clarify the effect of I~-OHD 3 on both senile and postmenopausal osteoporosis and to confirm the safety of this mode of therapy in a long-term basis in senile osteoporosis. In a previous study [2] we assessed the efficacy of this drug using single photon absorptiometry (SPA), which measures the bone mineral density in peripheral sites where cortical bone predominates. Thus, the question arises as to what bone sites respond to this mode of therapy in osteoporosis. Bone mineral densities of the lumbar vertebrae as well as the entire skeleton were therefore followed using dual-energy X-ray absorptiometry DXA) before and after the administration of 1 o~-OHD 3.