Ivermectin is a macrocyclic lactone that has widespread antiparasitic activity. Numerous clinical trials have shown that ivermectin is safe and effective in the treatment of human infection with Onchocerca volvulus. Although it is rapidly microfilaricidal, it does not cause a severe reaction, as is seen with diethylcarbamazine treatment. The drug temporarily interrupts production of microfilaria but has not known long-lasting effects on the adult worms. In patients with onchocerciasis, a single oral dose of ivermectin (150 micrograms/kg) repeated once a year leads to a marked reduction in skin microfilaria counts and ocular involvement. At this dose, ivermectin causes minimal side effects and is sufficiently free of severe reactions to be used on a mass scale. It promises to revolutionize the treatment of onchocerciasis.