In order to evaluate the effect of extracranial-intracranial (EC/IC) bypass surgery for the patients with ischaemic cerebrovascular disease, fourteen patients were studied for cerebral blood flow (CBF), oxygen extraction fraction (OEF) and cerebral metabolic rate of oxygen (CMRO2) before and after EC/IC bypass using positron emission tomography (PET) and 15O steady state technique. Six patients with carotid lesion received superficial temporal artery (STA)-middle cerebral artery anastomosis, and eight patients with vertebro-basilar lesion underwent STA-superior cerebellar artery anastomosis. In both groups of patients with carotid and vertebro-basilar lesion, significant increase of CBF and decrease of OEF were observed following surgical revascularization. Especially in patients with vertebro-basilar lesion, remarkable improvements were observed not only in the cerebellum but also in the cerebral cortex. In addition, the cases with preoperative higher OEF showed marked increase of CBF and decrease of OEF. It was suggested that EC/IC bypass can be effective when pre-operative uncoupling between cerebral circulation and metabolism are marked and PET study provides useful information for selecting the appropriate candidates for EC/IC bypass surgery.