Ivermectin, a recently developed macrocyclic lactone with broad antiparasitic activity, has been shown by a series of clinical trials to be 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. 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, although ivermectin has no known long-lasting effects on the adult worms. With treatment there is no significant exacerbation of either anterior or posterior segment eye disease even in those with severe ocular disease. Treatment leads to a marked and prolonged improvement in ocular status. Because of its safety and efficacy, ivermectin can be used on a mass scale and promises to revolutionize the treatment of onchocerciasis.