It is startling that the diagnosis of a disease on which so much professional effort and interest has been focused is suspected far more frequently than it is definitely made. A final diagnosis, even in the presence of typical clinical features, must depend on demonstration of the virus, either by animal inoculation or tissue culture techniques, as well as on changes in antibody titers between the acute and convalescent phases. Nevertheless, the diagnosis of poliomyelitis can be presumed with some confidence on purely clinical grounds as can the diagnosis of other viral diseases, such as measles or mumps, which is also most difficult to substantiate. In epidemics the suggestive spinal fluid findings and the typical spotty lower motor neuron type of paralysis may often justify an unequivocal clinical diagnosis. At times of high poliomyelitis incidence, however, the diagnosis of any vague febrile illness offers difficulties comparable to those of uncomplicated