Ten years of mass vaccination with oral poliovirus vaccine (OPV) have brought poliomyelitis under effective control in most parts of China and have greatly reduced its incidence. Outbreaks of poliomyelitis and poliomyelitis-like disease are occasionally reported in areas far from the big cities, however, and these outbreaks influence the total incidence in the country. Two causes have been suggested: (i) the Expanded Programme on Immunization was not strongly emphasized, with the result that the rate of immunization with OPV was low and that most susceptible children were not immunized; and (2) the rate of misdiagnosis was high, and paralytic cases caused by other enteroviruses were mistakenly diagnosed as poliomyelitis. Investigations of those outbreak areas by Dong [1] showed that 93% of poliomyelitis victims had never been immunized, that 1.4% had not been fully immunized, and that others had unclear vaccination histories. The results showed that the immunization rate must be improved. According to our experience, no matter how effective a vaccine, immunization of 90% of the susceptible population is necessary to prevent an outbreak. This requirement is perhaps important for the developing countries in which poliomyelitis is not yet controlled. For instance, in 1975 only one case was found in Shanghai City, where >90%o of the susceptible population was vaccinated. In the rural areas outside Shanghai, where the vaccination rate was 75%o-85qo, only one case was found in 1976.