OBJECTIVE To evaluate the effect of the hypoestrogenism induced by GnRH agonist (GNRH-a) therapy on cerebral vessel blood flow. METHODS Open, controlled study. METHODS Tertiary care units of the University of Milan, Italy. METHODS Young women scheduled to undergo 6 months of therapy with a GnRH-a; a control group was also enrolled. METHODS In both groups, the pulsatility index of both the internal carotid artery (ICA) and middle cerebral artery (MCA) was measured by means of Doppler ultrasound over a period of 6 months. METHODS The ICA and MCA pulsatility index. RESULTS No variation in the pulsatility index of either artery was found in either group. CONCLUSIONS A 6-month period of GnRH-a-induced hypoestrogenism in young women does not lead to any variation in the blood flow of cerebral vessels. This provides some reassurance as to the safety of these drugs in relation to the role that the reactivity of peripheral arteries may play in determining risk of cardiovascular disease. Furthermore, our results show that blood flow in the cerebral vessels of young subjects is under extraestrogenic control and that this may counterbalance estrogen deprivation through mechanisms that probably are no longer active in the perimenopausal years.