Cerebral post-ischemic hyperperfusion has been observed at the acute and subacute periods of ischemic stroke. In the animal stroke model, early post-ischemic hyperperfusion is the mark of recanalization of the occluded artery with reperfusion. In the PET studios of both humans and experimental animals, early post-ischemic hyperperfusion is not a key factor in the development of tissue infarction and indicates the spontaneous reperfusion of the ischemic brain tissue without late infarction or with small infarction. But late post-ischemic hyperperfusion shows the worse prognosis with reperfusion injury associated with brain tissue necrosis. Early post-ischemic hyperperfusion defined by PET and SPECT may be useful in predicting the prognosis of ischemic stroke and the effect of thrombolytic therapy.