There were seven clinical centers and a coordinating center in this randomized, double-blind clinical trial. The women were allocated to one of five treatment arms: placebo; conjugated equine estrogen (CEE), 0.625 milligrams per day; conjugated equine estrogen, 0.625 milligrams per day plus cyclic medroxyprogesterone acetate (MPA), 10 milligrams per day for 12 days per month; CEE, 0.625 milligrams per day plus consecutive MPA, 2.5 milligrams per day; CEE, 0.625 milligrams per day plus cyclic micronized progesterone (MP), 200 milligrams per day for 12 days a month. The four primary endpoints were chosen to represent four biological systems related to the risk of cardiovascular disease and included high density lipoprotein cholesterol (HDL-C), systolic blood pressure, serum insulin, and fibrinogen. Recruitment began in October 1989 and ended in February 1991. Baseline data collected included blood pressure, resting heart rate, weight, waist/hip ratios and endometrial biopsy. Laboratory evaluations included lipid panel, high density lipoprotein cholesterol, insulin and glucose, bone density, fibrinogen, and in three clinics, additional hemostasis factors, renin substrate, plasma renin activity, aldosterone, and oral post-heparin lipase activity. All women underwent an endometrial aspiration biopsy at baseline and annually thereafter. Additional biopsy specimens were obtained if there was noncyclic endometrial bleeding. All women also had baseline and annual mammograms. Other data were collected on quality of life, exercise, diet, alcohol use, and smoking. Participants were followed at three, six and twelve months post-randomization, and at six month intervals thereafter for three years. Post-trial analyses of existing data sets were funded for three years by the cooperative agreement mechanism beginning August 1, 1994. A three-year safety follow-up funded through the contract mechanism began in 1994. It included three annual visits at which endometrial biopsies, mammograms, and some limited health information were obtained.