Between 1917 and the late 1920s, encephalitis lethargica was an epidemic and often lethal neurologic disease. In adults, it typically elicited severe somatic effects, and in particular, various forms of cranial nerve and motor dysfunction. In children, the psychiatric effects were often as severe as the physical consequences. Approximately one third of affected children underwent a rapid transformation from normal behavior to delinquency, often leading to institutionalization. Many neurologic and psychological theories were advanced to explain these severe behavioral changes, and the therapeutic approaches employed ranged from training in dedicated schools to frontal leucotomy. Whereas epidemiologic associations provide both positive and negative support for an etiologic relationship between encephalitis lethargica and the approximately contemporaneous "Spanish" influenza epidemic, previously unutilized data from children provide some of the strongest links between influenza and encephalitis lethargica. Encephalitis lethargica triggered behavioral changes in children that are not duplicated by any other neurologic condition, with the possible exception of traumatic brain injury. These unique behavioral abnormalities may provide the earliest clear indication of new encephalitis lethargica cases, whether alone or in concert with an influenza epidemic.