Background Estrogen replacement therapy has been associated with a reduction in cardiovascular events in postmenopausal women. One of the mechanisms responsible may be a beneficial effect of estrogen on coronary vascular function. We therefore studied the short-term effects of estrogen on coronary artery dimensions and microvascular resistance in postmenopausal women.Methods and Results Twenty postmenopausal women 61+-7 years old participated in this study. Seven had angiographic evidence of atherosclerosis of the left coronary artery. Coronary artery diameters were measured by quantitative coronary angiography. Blood flow velocity was measured with a Doppler wire placed in a proximal left coronary artery segment. Left coronary artery infusions of acetylcholine (range, 10 sup -8 to 10 sup -5 mol/L estimated delivered concentrations) and of adenosine (n=18) and sodium nitroprusside (n=10) were performed before and during concomitant continuous intracoronary infusion of 17 beta -estradiol to test endothelium-dependent and independent vasodilation, respectively. Intracoronary infusion of estradiol increased coronary sinus estradiol levels from postmenopausal (16+-11 pg/mL) to premenopausal (282+-121 pg/mL) levels. Estradiol did not affect basal coronary artery diameter, blood flow, or resistance. Epicardial coronary artery constriction induced by acetylcholine infusion in the control study (maximum, 10+-15% from baseline) was prevented during repeat acetylcholine infusion with concomitant estradiol administration (P<.001). Estradiol potentiated the vasodilator coronary microvascular response to acetylcholine as manifest by significantly greater coronary flow (P<.001) and lower coronary resistance (P<.02). The reduction in coronary resistance from baseline in response to acetylcholine was significantly potentiated by estradiol (P=.01), with a mean decrease in coronary vascular resistance during acetylcholine infusion of 20+-38% before and 35+-33% during concomitant estradiol administration. The effect of estradiol on coronary dynamics was similar in women with and women without angiographically apparent left coronary artery atherosclerosis and was most prominent in women with the most impaired responses to acetylcholine at both the epicardial (r=-.72, P<.001) and microvascular (r=-.59, P=.006) coronary artery levels. In contrast, estradiol did not affect the coronary epicardial or microvascular vasodilator responses to adenosine or sodium nitroprusside.Conclusions Physiological levels of 17 beta -estradiol acutely and selectively potentiate endothelium-dependent vasodilation in both large coronary conductance arteries and coronary microvascular resistance arteries of postmenopausal women. This effect may contribute to the reduction in cardiovascular events observed with estrogen replacement therapy. (Circulation. 1994;89:2545-2551.)