Onchocerciasis is a major blinding disease in equatorial Africa and Central and South America. Ivermectin is a safe and effective drug in the treatment of this disease and now forms the basis of disease control in most endemic areas. We report the findings of long-term control of this infection in the Río Santiago focus in Ecuador, between January 1990 and December 1996, using a strategy of giving ivermectin treatments biannually in hyperendemic communities and annually in meso- and hypoendemic communities. Ivermectin was administered by local health workers from each community. A high level of compliance to ivermectin was achieved, with 81.9% to 98.0% of those eligible receiving the drug at each treatment instance. The impact of ivermectin therapy was monitored using a cohort of 120 randomly selected infected individuals from 8 hyperendemic communities. The geometric mean microfilarial density of this group declined from 19.3 to 0 mf/mg over the 84-month observation period. Ivermectin had a significant impact on anterior segment ocular disease, acute onchodermatitis and sowda. The rate of infection of blackflies declined from 1.1% in 1989-0.08% in 1996, which is below the vectorial capacity of the Simulium vector and, as no new nodules were detected after 1994 and no children under 5 became infected over the observation period, it is likely that the transmission of this infection was interrupted in the study area.