A 10-year double-blind prospective study was undertaken to evaluate the effects of estrogen replacement therapy (ERT). The sample population consisted of 84 pairs of randomly chosen postmenopausal in-patients, matched for age and diagnosis. The treatment group received high-dose conjugated estrogens, cyclically with progesterone. The controls received placebos. Results revealed no statistically significant difference in the incidence of thrombophlebitis, myocardial infarction (MI), or uterine cancer. There was a lower incidence of breast cancer in the treated group. Estrogen-treated patients showed a higher incidence of cholelithiasis. Those in the treated group who began the study with elevated beta/alpha lipoprotein ratios showed a reduction in that ratio over the course of the study, while the controls either maintained or increased their ratios. The low number of cases precludes drawing any real significance from the data on diseases of low frequency. The study excludes only a high incidence of complications from estrogens.