Abstract 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 < .001) and similarly during both regimens. This drop was already clearly detectable during the second month, from 0.97 (0.95, 1.01) (mean and 95% confidence intervals [CI]) to 0.94 (0.91, 0.97) in the oral and from 0.98 (0.94, 1.00) to 0.92 (0.89, 0.95) in the transdermal group, but it continued up to 12 months (0.85 [0.82, 0.88], 13% of baseline values in the oral group and 0.84 [0.81, 0.87], 14% in the transdermal group). In the uterine arteries, the drop in PI was steeper and greater and reached its maximum at 6 months (39% and 40%, respectively). Drops in carotid and uterine PI correlated positively with baseline PI values, but were not affected by patient age, time from menopause, previous HRT and smoking. Addition of norethisterone acetate did not counteract drops in carotid and uterine PI in either group. Conclusion: Oral and transdermal sequential HRT are similarly effective at 1 year in reducing impedance to flow in carotid and uterine circulation. This long-term vascular effect might explain how HRT protects women from cardiovascular disease.