A 10-year, double-blind prospective study was undertaken to evaluate the effects of estrogen replacement therapy. The sample population consisted of 84 pairs of randomly chosen postmenopausal patients who were matched for age and diagnosis. One half of the patients received conjugated estrogens and cyclic progesterone, while the other half received placebo. Estrogen-treated patients whose therapy started within 3 years of menopause showed improvement or no increase in osteoporosis. Control patients demonstrate an increase in their osteoporosis.