Onchocerciasis is one of the leading causes of blindness in the developing world. An estimated 40 million people are afflicted with this parasitic disease. World Health Organization vector control programs have had considerable success in interrupting the parasite transmission cycle in selected savanna regions of West Africa, but chemotherapeutic agents suitable for massive treatment campaigns have not been available. Controlled clinical studies have indicated that a single oral dose of ivermectin is safer and more effective therapy for onchocerciasis than the the standard seven- to 10-day course of diethylcarbamazine, the current drug of choice, and that ivermectin causes a more prolonged reduction in dermal microfilarial density. Patients treated with ivermectin are unable to infect the blackfly vector as long as the dermal microfilarial density remains low; therefore, once- or twice-yearly administration of ivermectin in community-wide therapy programs, either alone or in combination with vector control measures, may successfully interrupt transmission of the parasite and eventually eliminate the disease.