Conventional treatment with vitamin D, calcium, and estrogen will delay but not reverse osteoporosis. The addition of fluoride may increase bone mass but fails to increase bone strength; fracture incidence is actually increased in non-vertebral bone by fluoride. Clearly, successful treatment of osteoporosis remains an unsolved problem. In women, osteoporosis coincides with menopause. The hypothesis that progesterone and not estrogen is the missing factor was tested in a clinical setting and was found to be extraordinarily effective in reversing osteoporosis.