Not all poliomyelitis-like paralytic illnesses can be attributed to polioviruses. Among the most recently recognized agents able to cause severe central nervous system disease with persistent flaccid paralysis is enterovirus type 71. In the 1969-1973 California outbreaks during which strains of this type were first reported, meningitis predominated, but cases of encephalitis were also seen. Outbreaks in different regions of the world followed. In some, hand-foot-and-mouth syndrome predominated; in others, meningitis; and in some, the clinical patterns were mixed. In the 1975 Bulgarian outbreak of greater than 705 cases, 149 patients developed paralysis and 44 others died. Enterovirus 71 was incriminated as the cause of the epidemic, in which infants and young children were the chief victims. The 1978 Hungarian epidemic of central nervous system disease was mixed, involving a tick-borne encephalitis virus among adults and enterovirus 71 among children. This newly recognized enterovirus is also significant in relation to polio vaccines. At some time, vaccine inevitably will be given to persons already incubating enterovirus 71 infection, who then may present with the paralytic syndrome due to the wild enterovirus. Such cases could be mistakenly designated as related to polio vaccine, particularly since many enterovirus 71 strains are difficult to isolate under conditions that would readily yield poliovirus isolates from the vaccine.