We validated cerebrovascular reactivity (CVR) in Moyamoya disease measured by arterial spin labeling (ASL) simultaneously with 15O-positron emission tomography (PET) using hybrid PET/MRI scanner. We compared the three types of ASL including standard-delay, multi-delay with extended post-label delay, and velocity selective ASL (VS-ASL). To quantify absolute PET cerebral blood flow (CBF) without arterial blood samples, we scaled the static PET maps using simultaneously collected phase-contrast (PC) MRI. Multi-delay ASL showed the strongest correlation with PC-PET on both baseline and post-ACZ CBF (R2 = 0.48 and 0.66 respectively). Multi-delay ASL and VS-ASL showed similar good correlations with PC-PET on CVR.