Even with rapid development of other neuroimaging modalities such as MR imaging and CT, PET is the only technique that provides accurate, quantitative measurements of regional hemodynamics and metabolism in human subjects. Through the use of these combined measurements, we have greatly expanded our knowledge of the pathophysiology of cerebrovascular disease of different types. It has been possible to document the compensatory responses of the brain to reductions in perfusion pressure and to directly relate these responses to prognosis. PET measurements of OEF identify a subgroup of patients who have carotid occlusion and who are at increased risk for recurrent stroke who cannot be identified by any other clinical or arteriographic means. These measurements of OEF are being used to identify high-risk patients for inclusion in a clinical trial to assess the efficacy of surgical revascularization in reducing the subsequence of ipsilateral ischemic stroke. In acute ischemic stroke, attempts have been made to define the "ischemic penumbra" and to predict tissue viability and clinical outcome, although the reliability of PET markers of ischemia in distinguishing viable from irreversibly damaged tissue needs to be confirmed with independent data sets. Much work has been devoted to the investigation of the metabolic effects of infarcts and hemorrhages on remote areas of the brain; the clinical importance of such findings appears to be minimal. Early studies of recovery from stroke suggested functional reorganization of the brain, but further investigations with more rigorous experimental design need to be performed. Given the ease of performing such studies with functional MR imaging, it is likely that this technology will supplant PET for this specific indication. The importance of ischemia as a secondary mechanism of brain injury has been addressed in ICH and SAH. PET demonstrated that hematomas exert a primary depression of metabolism rather than inducing ischemia in the surrounding tissue. It also documented the integrity of autoregulation and provided clinically useful information regarding the safety of blood pressure reduction after ICH. Studies in SAH have differentiated the primary effects of the hemorrhage on cerebral hemodynamics and metabolism from those of vasospasm. PET studies are time-consuming, expensive, and require extensive facilities and technical support. In the field of cerebrovascular disease, PET has served as a specialized research tool at a few centers to help elucidate the pathophysiology of stroke. Up until now, however, PET scans in individual patients have not been demonstrated to be necessary for making patient care decisions. Whether the role of PET expands to impact the management of individual patients will depend on the results of investigations like the Carotid Occlusion Surgery Study that directly assess the ability of PET to influence patient outcome.