Cerebrovascular reactivity to carbon dioxide (CVR-CO2) is impaired in conditions affecting cerebral vasculature. Obesity increases the risk of Alzheimer’s disease (AD). The hippocampus plays a prominent role in cognition and it is one of the earliest brain structures affected during the progression of AD. It remains uncertain how obesity affects cerebral vasculature in AD vulnerable regions. We examined the relationship between body mass index and neocortical and hippocampal vasoreactivity. Our pulsed ASL sequence combined a flow-sensitive alternating inversion-recovery labeling scheme with balanced steady-state free precession to optimize spatial resolution and lower sensitivity to susceptibility artifacts. Cerebral blood flow (CBF) measurements were done during rest and rebreathing challenge designed to increased CO2 level. In obese women (BMI≥30, n=36) hippocampal vasoreactivity was 80% lower than in their non-obese peers. No relationship was observed in men or with respect to cortical vasoreactivity.