BACKGROUND–Hypertensive encephalopathy (HTE) is a syndrome typified by headache, seizures, and neurologic signs associated with increased systemic blood pressures; edema in the subcortical white matter is seen on imaging studies and is usually reversible, although infarction or hemorrhage may supervene. Based on previous work, we theorize that HTE is associated with increased perfusion to the brain. Syndromes related to HTE may also be encountered in clinical situations in which perfusion to the brain is acutely increased without systemic hypertension (i.e., after treatment of high-grade carotid stenoses or large intracranial arteriovenous malformations, or in high altitude mountain sickness). We therefore refer to these conditions more generally as hyperperfusion encephalopathies (HPE).REVIEW SUMMARY–The clinical and radiographic data of 110 patients (average age, 50.1 years) who presented at the Brigham and Women’s Hospital with clinical and radiographic signs of HPE were reviewed; 104 had systemic hypertension and 6 had postcarotid endarterectomy hyperperfusion syndrome. Edema involved the subcortical white matter and occasionally the cortex in all patients. In patients with systemic hypertension, the edema was usually bilateral and located predominantly in the occipital lobes; other brain regions included the parietal lobes, posterior frontal lobes, cerebellum, and splenium of the corpus callosum. The six patients with postcarotid endarterectomy hyperperfusion syndrome had edema in the hemisphere ipsilateral to the operated side involving the anterior and middle cerebral artery territories. The edema in HPE was associated with: increased low attenuation on CT; decreased T1and increased T2signal on MR imaging; increased cerebral perfusion on single emission computed tomography (SPECT) and perfusion MR imaging; and did not show restricted diffusion on MR imaging. The syndrome resolved completely in most cases after the administration of antihypertensive agents, although rarely small infarcts and hemorrhages occurred. Three patients with thrombocytopenia developed large fatal intracranial hemorrhages.CONCLUSION–The symptoms of HPE are usually nonspecific, but the radiographic findings are consistent. Treatment should be instituted rapidly and patients should be followed until the condition resolves either clinically or radiographically; hemorrhagic complications, although rare, can be serious.