One hundred and twenty patients out of 220 in a previous study were retreated after 6 months or one year with doses of 100, 150 or 200 mcg/kg of ivermectin. The tolerance was excellent due probably to the small number of skin microfilariae obtained with the first treatment. The annual treatment with 150 to 200 mcg/kg, better than with 100 mcg/kg, keeps for the year following the second treatment the microfilariae number between 6 and 11% of the initial level. The half-yearly administration keeps it between 1 and 7% and especially 94 to 100% of these retreated patients have a level of microfilariae less than 5 mf/mg. The results obtained with the half-yearly treatment show a considerable reduction of the number of microfilariae in the anterior chamber of the eye and the percentage of positive patients. Ivermectin is a very efficient microfilaricidal drug for the treatment of onchocerciasis and the prevention of ocular complications. Its rational use in mass campaigns should reduce, if not interrupt, the transmission of this parasitic disease.