Epidemic encephalitis has become an increasingly important cause of insanity among the population, and during the past few years this disease has been seen more and more among the new admissions of mental hospitals. The various forms in which this disease may manifest itself in the chronic stage are now well known, and can be intimately studied in any large institution. Up till the present, however, no account has yet been reported, so far as I am aware, of the appearance of encephalitis lethargica among patients already resident in a mental hospital.