Postmenopausal bone estrogen deficiency causes rapid bone loss. Therefore hormone replacement therapy (HRT) has been considered as a rational therapy for postmenopausal not only for osteopenia but also osteoporosis. The results of Women's Health Initiative (WHI) hormone trial had given the strong restriction on HRT. However recent closer analyses have shown that HRT initiated for early menopausal women brings many benefits without elevation of the risks, and that the appropriate selection of dose level or a way to apply can also reduce the risks of HRT. HRT has many physiological effects that were not seen in other drugs and so HRT can be considered as one of the most effective treatment for postmenopausal osteoporosis.