ObjectiveTo examine the effects of oral and transdermal estrogen replacement therapy (ERT) on ambulatory 24-hour blood pressure (BP) recordings.MethodsIn a nonrandomized, prospective study, 90 normotensive, oophorectomized women, ages 30–59 years, underwent ambulatory 24-hour BP measurements at study entry and after 3 and 6 months of either oral (n= 50) or transdermal (n= 40) ERT.ResultsIn the women receiving transdermal estrogen, we observed a change in mean nighttime systolic BP of −4.2 mmHg (95% confidence interval [CI] −7.7, −0.7;P= .039) after 6 months' treatment. There was a change in mean daytime diastolic BP after 3 months (−3.3 mmHg; 95% CI −5.5, −0.9;P= .016) and 6 months (−4 mmHg; 95% CI −6.8, −1.2;P= .014), and in mean nighttime diastolic BP after 3 months (−3.8 mmHg; 95% CI −6.6, −0.9;P= .027) and 6 months (−4.4 mmHg; 95% CI −7.1, −1.7;P= .005). No significant BP changes were observed in the women taking oral estrogen. Although the statistical power to detect a change of 4 mmHg at the 5% significance level was 90% for diastolic BP, it was weaker for systolic BP (63%) in this group. However, in more than one-third of the women receiving either treatment, a statistically significant increase in BP was observed.ConclusionsTransdermal ERT was associated with a reduction in mean ambulatory BP, whereas oral treatment did not after BP. Although the overall effect of estrogen was to lower BP, individual responses were variable, and BP increased in more than one-third of the women on either treatment. Therefore, long-term monitoring of ambulatory measurements may be required.