This study aimed to elucidate the effects of change in estrogen during the menstrual cycle and menopause on shear-mediated dilation of the internal carotid artery (ICA), a potential index of cerebrovascular endothelial function. Shear-mediated dilation of the ICA and serum estradiol were measured in 11 premenopausal (Pre-M, 21±1yrs), 13 perimenopausal (Peri-M, 49±2yrs), and 10 postmenopausal (Post-M, 65±7yrs) women. Measurements were made twice within the Pre-M group at their early follicular (EF, lower estradiol) and late follicular (LF, higher estradiol) phases. Shear-mediated dilation was induced by 3min of hypercapnia (target PCO +10mmHg from individual baseline) and was calculated as the percent rise in peak diameter relative to baseline diameter. ICA diameter and blood velocity were simultaneously measured by Doppler ultrasound. In Pre-M, shear-mediated dilation was higher during the LF phase than during the EF phase (P<0.01). Comparing all groups, shear-mediated dilation was reduced across the menopausal transition (P<0.01), and Pre-M during the LF phase showed the highest value (8.9±1.4%) compared with other groups (Pre-M in EF, 6.4±1.1%; Peri-M, 5.5±1.3%; Post-M, 5.2±1.9%, P<0.05 for all). Shear-mediated dilation was positively correlated with serum estradiol even after adjustment of age (P<0.01, r=0.55, age-adjusted; P=0.02, r=0.35). Collectively, these data indicate that controlling the menstrual cycle phase is necessary for the cross-sectional assessments of shear-mediated dilation of the ICA in premenopausal women. Moreover, current findings suggest that a decline in cerebrovascular endothelial function may be partly related to the reduced circulating estrogen levels in peri- and postmenopausal women.