Large numbers of febrile illnesses associated with a great variety of clinical manifestations occur among children and adults each summer and autumn in temperate zones. There is increasing evidence that the family of enteroviruses,1 comprising the polioviruses, Coxsackie, and ECHO viruses, are responsible for a large proportion of these seasonal diseases. Since we already know of 51 antigenically distinct types in the family of enteroviruses, with more candidates awaiting their turn for admission, it seems highly desirable to learn of the clinical and epidemiologic potentialities of each virus. Such information can best be obtained during epidemics caused predominantly by one virus, and then only with clinical and epidemiologic data based exclusively on virologically proved cases. The present communication contains the results of such a study of an epidemic of ECHO Type 9 virus disease during the summer of 1957 in the city of Milwaukee, with an estimated population of