Positron emission tomography (PET) has contributed to the solution for the pathophysiology of the ischemic brain with defining the ischemic penumbra, cerebral perfusion reserve, and uncoupling state between cerebral blood flow (CBF) and cerebral oxygen metabolism. Recently, 15O-PET study in the hyperacute stage of cerebral infarction has elucidated the deterioration of oxygen metabolism in the areas with infarct growth, which phenomenon was called as ‘metabolic penumbra’. New PET imaging of the peripheral benzodiazepine receptor has proved the increase of the activated microglia in the areas with metabolic penumbra, and it may have a potential to become a biomarker of the pharmacological effect in this area. Brain energy metabolism has reevaluated on the basis of the relationship between neuron and astrocyte, and an important role of the astrocyte in the lactate production and recycle of glutamic acid has been reported. 11C-labeled acetate may provide information of TCA cycle of the astrocyte and the activation of glutamine-glutamic acid cycle. PET study for the cerebrovascular disease stepped into a new era by using a novel tracer for molecular imaging.