Estrogen appears to enhance cerebral blood flow (CBF). An association between cerebral blood flow and physiologically altered hormonal levels due to menstrual cycle, menopause or exogenous manipulations such as ovariectomy or hormone replacement therapy, has been demonstrated. The purpose of this study was to determine the association between ovarian stimulation and CBF in-vivo by measuring blood flow in the internal carotid artery following pituitary suppression and during controlled ovarian stimulation in women undergoing in-vitro fertilization treatment cycles. Internal carotid artery (ICA) volume flows were measured by dual-beam angle independent ultrasound Doppler in 12 women undergoing controlled ovarian stimulation. Measurements were performed following pituitary/ovarian suppression, in the late follicular phase, and at mid luteal phase. Blood flow in the ICA increased by 22.2% and by 32% in the late follicular and mid-luteal phase compared to the respective values obtained during ovarian suppression (p<0.0005 and p<0.0001, respectively). There was a significant correlation between increments in estrogen levels and increments in CBF when comparing the late-follicular phase with the ovarian suppression period (r=0.8, p<0.001). Mean blood flow velocity significantly increased (by 15.7% and 16.9% respectively) and cerebral vascular resistance significantly decreased (by 17.6% and 26.5%) during the late-follicular and the mid-luteal phases compared to respective measures during ovarian suppression. There was a significant correlation between an increase in estrogen levels and a decrease in cerebral vascular resistance when comparing the late-follicular phase with the ovarian suppression period (r = -0.6, p<0.05). These changes imply sex hormones-associated intracranial vasodilatation leading to increased CBF during controlled ovarian stimulation