To investigate the possible existence of chronic selective hemodynamic impairment in the arterial borderzone regions of the brain, we used positron emission tomography (PET) to measure regional mean vascular transit time (rt, equal to the ratio of regional cerebral blood volume to cerebral blood flow) and regional oxygen extraction fraction (rOEF) in 32 patients with either severe internal carotid artery stenosis or occlusion and 11 normal controls. Twenty-four of the patients had had TIAs or amaurosis fugax from 1 to 60 days before PET; all had normal brain CT. We used a stereotactic localization method to locate the anterior and posterior borderzone regions of the middle cerebral artery (MCA) territory. We then calculated ratios of each borderzone to the ipsilateral MCA territory for both rt and rOEF. There was no significant difference from control ratios in any patient subgroup including those with greater than or equal to 75% stenosis or occlusion, those with or without contralateral greater than or equal to 50% stenosis, or those with abnormal hemodynamics in the MCA territory. We therefore found no evidence for selective borderzone hemodynamic impairment in this group of patients with severe carotid artery disease.