Ivermectin is a synthetic derivative of a macrocyclic lactone produced by an actinomycete Streptomyces avermitilis. It has a broad spectrum antiparasitic activity against nematodes and certain acarians in animals. The microfilaricide action of this product against horse and cattle onchocercosis led to the study of its effects in human onchocercosis against O. volvulus. Several trials performed mainly in endemic zones of Africa showed that this drug was more effective than the reference microfilaricide, diethylcarbamazine. A single oral dose of 200 micrograms/kg of Ivermectin reduces the dermal microfilaria population to nearly zero within a few days and the effect is maintained for at least 6 months. Secondary ocular or systemic effects are rare, negligible and transitory. The prolonged elimination of dermal microfilariasis caused by sequestration followed by degeneration of the microfilaria in the uterus of females raises the hope that Ivermectin used in a single annual or bi-annual dose will contribute to the interruption of the transmission of this serious parasitic disease.