We report six previously healthy children who several days after a prodromal illness had an acute encephalopathy that ran a biphasic course. It appears to constitute a recognizable syndrome with a good prognosis that can be differentiated from other encephalopathies of obscure origin as previously defined by Lyon et al. The active phase was dominated by coma or confusion and by abnormal movements, including disordered gesticulation and attacks of orofacial dyskinesia or limb dystonia associated with permanent rigidity and culminating in opisthotonic posturing. Repeated seizures were observed in only two patients. Permanent slow waves were recorded on the electroencephalogram in all patients, even during bursts of abnormal movements. Cerebrospinal fluid and results of serologic studies were normal throughout the course of the disease, and attempts at viral isolation and antiviral antibody detection yielded negative results. Brain imaging either showed no abnormalities or suggested a moderate degree of brain edema. The recovery phase, which extended for several weeks, was characterized by a rapid return of motor function and persistent behavioral and cognitive disturbances. Nonverbal reasoning recovered long before verbal expression returned to normal. Four patients eventually recovered fully, whereas two had mild sequelae.