The World Health Organization recommends conducting supplemental immunization activities to eradicate poliomyelitis by the year 2000. Although effective in eliminating poliomyelitis from the Americas, supplemental campaigns require substantial resources. To assess differential campaign effectiveness in eliminating this disease, poliomyelitis occurrence was compared in counties in China that targeted children <3 versus <4 years of age. Counties that targeted children <3 years of age reported poliomyelitis more frequently after the campaigns. This association was observed even after accounting for the effects of previous poliomyelitis occurrence, urban versus rural setting, and population density. While several limitations emphasize the preliminary nature of these findings, these data support targeting the widest possible age group of susceptible children to ensure maximum effectiveness in eliminating poliomyelitis. Thus, while reducing the target age of these activities may result in considerable resource savings, such campaigns may not be as effective in eliminating poliomyelitis.