OBJECTIVE The argument about why the head-up tilt table test (HUT) does not include the posterior cerebral circulation, which is mainly responsible for syncope, as a monitor target has not been resolved. It is also unclear whether there is a sex difference in cerebral blood flow (CBF) changes. We hypothesized that orthostatic CBF changes more in the posterior circulation than in the anterior circulation and is different between sexes. METHODS Thirty healthy volunteers (13 female and 17 male) were recruited for the HUT. The blood pressure (BP), middle cerebral artery flow velocity (MCAFV), and posterior cerebral artery flow velocity (PCAFV) were monitored simultaneously. Static cerebral autoregulation (CA) was calculated. RESULTS The female volunteers had a lower BP, but there was no difference in orthostatic BP changes (female versus male: 1.29% +/- 5.26% versus 4.22% +/- 12.65%; P = .65). The female volunteers had a significantly greater orthostatic drop in the PCAFV than in the MCAFV (23.8% +/- 9.1% versus 18.2% +/- 7.3%; P = .008). The static CA in the middle cerebral artery was better than in the posterior cerebral artery, although not significantly (13.6% +/- 34.8% versus - 2.8% +/- 12.2%; P = .15). CONCLUSIONS Our study showed the different cerebral hemodynamic responses between anterior and posterior circulations and between sexes during the HUT. We conclude that HUT studies for syncope should include the posterior cerebral circulation, especially for female patients.