Three women (aged 21-36 years) developed acute illnesses that were similar to epidemic encephalitis lethargica. Each presented with a neuropsychiatric disturbance that was succeeded by pyrexia, a fluctuating conscious state and involuntary movements including oculogyria. Cerebrospinal fluid examination showed a predominantly lymphocytic pleocytosis (64-120x10(6) cells/L) and oligoclonal bands were detected in two cases. Two patients died, while the third made a gradual recovery. Post-mortem examination in the two fatal cases showed changes of lymphocytic meningitis and focal diencephalic lymphocytic infiltration, although these changes were mild relative to the effects of the clinical illness. The diagnosis of sporadic encephalitis lethargica relies on identifying shared clinical features with the past epidemic disease plus circumstantial evidence of immunological activity from laboratory investigations and some tests of exclusion of other disorders.