Initial clinical trials with ivermectin were performed in patients with both roundworm infestation and onchocerciasis. Obvious clinical safety allowed for rapid progression through 5-30-50-100-150-200 mcg/kg in infected patients. Initial studies showed some effect at 50 mcg/kg; subsequent double-blind controlled studies, either with placebo or diethylcarbamazine (DEC), confirmed the efficacy of ivermectin as well as further defining its safety profile. Absence of adverse eye findings or serious systemic reactions justified the further open trials. Studies of patients treated at 6, 12, or 18 month intervals showed a long lasting effect of ivermectin in reducing skin microfilaria counts. Phase III studies confirmed safety and efficacy and further refined the dose to 150 mcg/kg every 12 months. Large trials in Liberia and other countries in West Africa, and subsequently under Onchocerciasis Control Program (OCP), included approximately 120,000 persons carefully followed during which few patients with serious adverse experiences were reported. These extensive field trials confirmed the relative safety allowing for broad distribution of ivermectin in programs not able to provide physician monitoring.