Onchocerciasis is a major blinding disease that, until recently, has been essentially untreatable. Ivermectin is a safe and effective drug for the mass treatment of onchocerciasis and when used on an individual basis, it reduces the ability of the treated person to transmit Onchocerca volvulus infection. In the present study, the effect of community-based ivermectin treatment on the degree of transmission within the community was assessed by determining the incidence of new infection in children. Ivermectin was distributed annually on three occasions to the eligible members of a population of approximately 14,000 people living on a rubber plantation in a forest area endemic for onchocerciasis. After 2 years, the prevalence of infection in 5-year-old children decreased by 21%. The annual incidence in an uninfected cohort of children decreased by 35% and, after age-specific adjustment, the reduction in incidence in 7- to 12-year-old children was 45%. Thus, community-based distribution of ivermectin led to a significant reduction in incidence of new infection. These findings suggest that ivermectin can be important in reducing the transmission of onchocerciasis.