In the summer of 1987 five children were seen at The Children's Hospital of Philadelphia because of acute onset of flaccid paralysis of an arm or leg(s). Although there were documented exposures to oral poliovirus vaccine and coxsackievirus B3 in some of the cases, the clinical, epidemiologic and laboratory findings indicate that enterovirus 71 was the common etiologic agent for this unusual outbreak of poliomyelitis-like paralysis. Of the five children three recovered completely; the other two had residual paralysis with weakness and muscle wasting. Imaging studies of the spinal cord in the two children with residual paralysis revealed defects in the ventral aspect of the spinal cord. This series of paralytic cases attributed to enterovirus 71 is the largest reported in the United States.