The importance of hemodynamic factors in the pathogenesis and treatment of ischemic cerebrovascular disease is not clear. We have investigated the relationship between cerebral hemodynamics and the subsequent risk of stroke in 30 medically treated patients with symptomatic occlusion or greater than 75% intracranial stenosis of the carotid arterial system. Positron emission tomography (PET) was used to evaluate the regional hemodynamic status of the cerebral circulation. Clinical follow-up to 1 year post-PET was available for all patients. The incidence at 1 year of all strokes was 1/9 for patients with normal hemodynamics and 1/21 for patients with abnormal hemodynamics. The 1-year incidence of ipsilateral ischemic stroke was 1/9 for hemodynamically normal patients and 0/21 in the abnormal group. The 21 patients in the abnormal group fulfilled entry criteria for the Extracranial-Intracranial Bypass Trial. The 0/21 incidence of ipsilateral ischemic stroke at 1 year was compared with the 1-year rate of 0.109 for the 714 medically treated patients from the Bypass Trial. We were able to reject with better than 90% certainty (p = 0.089) the hypothesis that our sample of patients came from a population with an ipsilateral ischemic stroke rate of 0.109 or greater. Thus, in this small sample, we found no evidence that PET evidence of abnormal cerebral hemodynamics identifies a subgroup of patients at higher risk for early stroke if treated medically with antithrombotic drugs.