PURPOSE: The purpose of this study was to evaluate whether cerebral perfusion from bypassed arteries can be demonstrated on regional perfusion imaging (RPI) using arterial spin labeling. We then compared cerebral perfusion on RPI and digital subtraction angiography (DSA) in moyamoya patients who underwent extracranial-intracranial bypass surgery. MATERIALS AND METHODS: We performed RPI using a 3-T magnetic resonance scanner and DSA studies in 11 moyamoya patients treated by bypass surgery. For RPI we placed a selective labeling slab on the bypassed external carotid artery. Two neuroradiologists determined the extent and location of the cerebral perfusion from bypass arteries in the middle cerebral artery territories on RPI and DSA. Kappa analysis was used to assess the interobserver agreement with respect to the extent and location of the cerebral perfusion and to evaluate the intermodality agreement between RPI and DSA. RESULTS: Interobserver agreement for the extent of cerebral perfusion on RPI was very good (kappa=0.89), with excellent location (kappa=1.00). Intermodality agreement for the extent of perfusion was very good (kappa=0.89), with good location (kappa=0.74). CONCLUSION: RPI is useful for evaluating cerebral perfusion from bypass arteries in moyamoya patients.