Estrogens are well established as agents that stabilize the skeleton and reduce the risk of osteoporotic fractures among postmenopausal women. For maximum benefit, preventive therapy should begin as early as possible after ovarian failure begins to occur. Efforts to prevent bone loss are likely to achieve the best results when initiated prior to significant loss of bone tissue and trabecular penetration. An effect on skeletal bone mass can be obtained by any route of administration and transdermal estrogen use is an alternative to oral estrogen. Long-term therapy may reduce the risk of hip fracture by 50% and of vertebral fracture by a greater amount. The minimum effective dose is probably that which achieves circulating estrogen levels in the mid-follicular range. For women with a uterus in place, a progestin usually is provided to protect the endometrium; it is given cyclically in younger women but may be given continuously in women several years past menopause. Progestins do not interfere with the effects of estrogen on the skeleton, and it is possible that some progestins enhance the skeletal effects of estrogen. For patients with osteoporosis, estrogens can be used as first-line therapy since in these patients they have the same skeletal stabilizing effect and reduce the risk of recurrent fracture.