The safety, acceptability, and efficacy of ivermectin during community-based mass treatment of onchocerciasis was assessed. Ivermectin was distributed three times, 1 year apart, to the population of a rubber plantation (14,000 people) in Liberia where greater than 80% of adults have Onchocerca volvulus infection. In a sample of adults, the microfilarial density was reduced by 84% after 2 years of treatment. Acceptance was as high as 98% during the third year. Adverse reactions occurred in 6-13 in 1000 people after first treatment and in only 3-4 in 1000 receiving their second or third annual dose of ivermectin; severe adverse reactions were not seen. These data show that community-based treatment with ivermectin is safe, well accepted, and effective in reducing microfilarial loads and suggest that ivermectin is the first practical drug suitable for mass treatment campaigns to control human onchocerciasis.