The type of encephalitis that was epidemic in St. Louis during the summer of 1933 differed materially from the popular conception of epidemic, or lethargic, encephalitis as heretofore observed in this country. Not only was there an unprecedented number of cases occurring within a short period of time and in a restricted geographic area, but the course and symptomatology of the disease were unlike those commonly observed in the older variety of encephalitis. In brief, the St. Louis cases were characterized by a rather sudden onset, with headache, high fever, stiff neck, mental confusion, tremors and other neurologic symptoms, but only occasional transient ocular manifestations. Following a stormy course, in most instances rapid improvement occurred, leading to apparently complete recovery within two or three weeks and leaving no residual effects. The present report is based on a study of the records of 786 hospitalized patients, and personal observation of approximately