The main challenge of vaccine control of poliomyelitis in the 1980s is in the subtropical and tropical regions of the world where "lameness" surveys in recent years have shown how very high the average annual incidence of paralytic poliomyelitis can be in both rural and urban areas in the absence of epidemics. The procedures by which oral polio vaccine (OPV) rapidly eliminated all or almost all paralytic disease caused by polioviruses from the economically developed temperate climate countries have been inadequate in tropical and subtropical countries, except in some small countries with good health services, largely because there is much more year-round circulation of "wild" polioviruses which continue to produce the disease in the unvaccinated and incompletely vaccinated children. Not even a cheap, hypothetically 100 percent effective, one-dose vaccine could eliminate poliomyelitis in the tropics if, for a variety of reasons, it would reach only a portion of the infant population. Paralytic disease caused by polioviruses has been quickly eliminated from both small and large tropical countries by OPV in well-organized programs of annual mass vaccinations of almost all children under a certain age.