OBJECTIVE Our purpose was to determine whether sex steroids alter aortic size and compliance in postmenopausal women. METHODS Twenty-six postmenopausal women were randomized to receive either conjugated estrogens 0.625 mg per day (group 1) or conjugated estrogens 0.625 mg per day and medroxyprogesterone 2.5 mg per day (group 2). Aortic cross-sectional area was measured by magnetic resonance imaging before and after 3 months of hormone therapy. RESULTS Estradiol levels increased in both group 1 and group 2 (p < 0.0001). Ascending aortic cross-sectional area increased from 439 +/- 7 mm2 to 466 +/- 7 mm2 in group 1 (p < 0.008) but was unchanged in group 2. Within the range of aortic pressures studied, no change in aortic compliance could be detected. CONCLUSIONS Estrogen therapy in postmenopausal women was associated with an increase in aortic size; but this effect was not detectable with the addition of progestin. The potential antagonistic effect of progestin on estrogen-induced aortic enlargement suggests that the favorable cardiovascular effects of postmenopausal estrogen therapy cannot be automatically extended to the combination estrogen-progestin.