Unstable angina is a frequent diagnosis in post-menopausal women and is associated with a significant risk of myocardial infarction and need for revascularization. The pathogenesis of unstable angina involves vasoconstriction superimposed on fixed disease, causing a temporary decrease in coronary blood flow. Recent catheterization studies in patients with atherosclerosis utilizing quantitative angiography and intracoronary doppler measurements of blood flow velocity suggest that endothelial dysfunction results in a paradoxical coronary vasoconstriction response to certain neurohumoral stimuli including acetylcholine, catecholamines, and serotonin with resultant myocardial ischemia. Therapeutic agents which prevent or limit this vasoconstriction may prevent recurrent ischemia and/or myocardial infarction in unstable angina patients. Recently, estrogen receptors were identified in the smooth muscle of post-mortem human coronary arteries. Work in animal models and studies in post-menopausal women suggest that intravenous estrogen acutely decreases coronary vascular resistance, increases coronary blood flow, and prevents the paradoxical response to acetylcholine in patients with endothelial dysfunction.