Vascular inflammation plays an important role in the pathogenesis of atherosclerosis and may contribute to stiffening of arteries that increases the risk of myocardial infarction and stroke. Accordingly, therapies that reduce vascular inflammation may reduce cardiovascular risk. The effect of estrogen therapy on serum markers of inflammation in postmenopausal women is divergent: Estrogen increases the levels of C-reactive protein, interleukin-6, and matrix metalloproteinase-9 ( MMP-9), but decreases levels of the soluble cell adhesion molecules ICAM-1, VCAM-1 and E-selectin. MMP-9 is secreted by macrophages and cytokine-activated smooth muscle cells, with increased expression in the vicinity of atherosclerotic plaques. Although activation of MMP-9 by estrogen could be deleterious in women with coronary artery disease by digesting the fibrous caps of vulnerable plaques and provoking thrombosis (consistent with the Heart and Estrogen/progestin Replacement Study), activation of MMP-9 in healthy postmenopausal women may remove excess matrix proteins that contribute to arterial stiffness (reduced compliance), thus reducing cardiovascular risk (consistent with the Nurses' Health Study). Reduction of levels of cell adhesion molecules might protect against new plaque development in both groups of women. The purposes of this protocol are to determine the net effects of estrogen therapy on 1) vascular inflammation in postmenopausal women, using MRI/MRA imaging of the carotid arteries, and 2) arterial compliance, derived from carotid ultrasonography and blood pressure measurements.