The Heart and Estrogen/progestin Replacement Study (HERS) is a randomized, double-blind, placebo-controlled trial designed to test the efficacy and safety of estrogen plus progestin therapy for prevention of recurrent coronary heart disease (CHD) events in women. The participants are postmenopausal women with a uterus and with CHD as evidenced by prior myocardial infarction, coronary artery bypass graft surgery, percutaneous transluminal coronary angioplasty, or other mechanical revascularization or at least 50% occlusion of a major coronary artery. Between February 1993 and September 1994, 20 HERS centers recruited and randomized 2763 women. Participants ranged in age from 44 to 79 years, with a mean age of 66.7 (SD 6.7) years. Most participants were white (89%), married (57%), and had completed high school or some college (80%). As expected, the prevalence of coronary risk factors was high: 62% were past or current smokers, 59% had hypertension, 90% had serum LDL-cholesterol of 100 mg/dL or higher, and 23% had diabetes. Each woman was randomly assigned to receive one tablet containing 0.625 mg conjugated estrogens plus 2.5 mg medroxyprogesterone acetate daily or an identical placebo. Participants will be evaluated every 4 months for an average of 4.2 years for the occurrence of CHD events (CHD death and nonfatal myocardial infarction). We will also assess other major CHD endpoints, including revascularization and hospitalization for unstable angina. The primary analysis will compare the rate of CHD events in women assigned to active treatment with the rate in those assigned to placebo. The trial was designed to have power greater than 90% to detect a 35% reduction in the incidence of CHD events, assuming a 50% lag in effect for 2 years and a 5% annual event rate in the placebo group. The design, analysis, and conduct of the study are controlled by the Steering Committee of Principal Investigators and coordinated at the University of California, San Francisco. HERS is the largest trial of any intervention to reduce the risk of recurrent CHD events in women with heart disease and is the first controlled trial to seek evidence of the efficacy and safety of postmenopausal hormone therapy to prevent recurrent CHD events.