To evaluate the changes of regional cerebral blood flow (CBF) in artificial hypertensive state, we investigated vascular responses to changes of arterial carbon dioxide tension (PCO2) and blood pressure by means of positron emission tomography (PET). The subjects were eight brain tumors which were histologically proven. We calculated CBF changes in hypercapnic and hypocapnic state, and then using those results, CBF changes in hypertensive state were corrected. Every patient represented a selective increase of tumor blood flow in hypertensive state induced by angiotensin II, demonstrating a loss of autoregulation in tumor. Our results suggested the possibility of enhancement of chemotherapy for brain tumors.