OBJECTIVE: To compare the long-term effects of oral and transdermal hormone replacement therapy (HRT) on carotid and uterine vascular impedance. METHODS: Sixty-three postmenopausal women were randomized to 1 year's treatment with oral or transdermal sequential combined HRT. Carotid and uterine artery pulsatility indices (PIs) were assessed by color Doppler at baseline, and after 2, 6, and 12 months of treatment. Fifty-eight women completed the trial, 27 in the oral and 31 in the transdermal group. In a subgroup of 30 women, we also performed Doppler measurements in the estrogen-progestin combined phase. The study had 90% power to detect a difference between treatment groups of 0.05 in the carotid artery and of 0.25 in uterine artery PI at the 5% significance level. RESULTS: The carotid PI decreased significantly (P