Postmenopausal osteoporosis, the most common bone disease in the developed world¹, is associated with estrogen deficiency. This deficiency induces increased generation and activity of osteoclasts, which perforate bone trabeculae, thus reducing their strength and increasing fracture risk. Estrogen replacement prevents these effects², indicating that estrogen negatively regulates osteoclast formation and function, but how it does this is unclear. Because functional osteoclast life span and thus the amount of bone that osteoclasts resorb could also be enhanced following estrogen deficiency, and since sex steroids regulate apoptosis in other target tissues³, we investigated whether estrogen may affect osteoclast function by promoting apoptosis. 17β–Estradiol promoted apoptosis of murine osteoclasts in vitro and in vivo by two– to threefold. Tamoxifen, which has estrogenic effects on bone resorption⁴, and transforming growth factor–β1 (TGF–β), whose production by osteoblasts is increased by estrogen⁵, had similar effects in vitro. Anti–TGF–β antibody inhibited TGF–β–, estrogen– and tamoxifen–induced osteoclast apoptosis, indicating that TGF–β might mediate this effect. These findings suggest that estrogen may prevent excessive bone loss before and after the menopause by limiting osteoclast life span through promotion of apoptosis. The development of analogues to promote this mechanism specifically could be a useful and novel therapeutic approach to prevent postmenopausal osteoporosis.