Rasmussen's encephalitis is a devastating disease of childhood causing progressive neurologic deficits and intractable seizure activity. Patients frequently have episodes of epilepsia partialis continua and, much less frequently, generalized status epilepticus. The seizures are intractable despite aggressive medical management. In advanced cases, hemispherectomy appears to be the only option to control the seizures. Permanent physical and mental impairments are inevitable. The cause of this disease is unknown, although pathologic specimens demonstrate nonspecific changes that are compatible with viral encephalitis. The progressive brain damage is typically so insidious in onset and gradual in course that it is difficult to make an accurate diagnosis on the basis of clinical evidence. We retrospectively evaluated the CT, xenon CT, positron emission tomographic, and MR neuroimaging findings in four young patients with pathologically suspected Rasmussen's encephalitis (three patients had CT scans, two had xenon CT scans, two had MR scans, and one had a positron emission tomogram). All studies showed abnormalities of the involved cerebral hemisphere: CT and MR revealed nonspecific atrophy, xenon CT showed decreased cerebral blood flow, and positron emission tomography revealed a hypometabolic state. Rasmussen's encephalitis is a diagnosis of exclusion; however, the information obtained from neuroimaging studies in combination with the clinical course should suggest this disorder.