Onchocerciasis continues to be a major cause of blindness, particularly in those sub-Saharan African countries which are outside the area of West Africa monitored by the Onchocerciasis Control Programme (OCP). Onchocercal ocular disease and blindness develop as a result of long exposure to onchocercal infection. Until 1987, suramin and diethylcarbamazine were the only drugs available for the treatment of onchocerciasis and they could not be used for community therapy because of their toxicity and the dosage schedules required. The registration of Mectizan (ivermectin, MSD) for treatment of human onchocerciasis in 1987, and the donation of this drug by Merck & Co. for as long as it is needed, provided a new opportunity for the safe treatment and control of the disease. The data available on the impact of repeated doses of Mectizan on ocular onchocercal disease indicate a significant reduction of ocular microfilarial loads and regression of early lesions of the anterior segment, including iridocyclitis and sclerosing keratitis. Such improvements are seen more rapidly when Mectizan is used than when onchocerciasis is limited by vector control alone. Mectizan treatment also has a beneficial effect on onchocercal optic-nerve disease and visual-field loss. Long-term maintenance of Mectizan therapy should lead to a reduction in the prevalence of blindness in endemic communities.