In an open clinical trial in phase IV, 856 onchocerciasis infected subjects received 150 micrograms/kg of ivermectin in May 1987. While 607 were included as witness. This cohort was revisited 7 and 12 months after. In June 1988, the same treatment was administrated to the previously treated subjects, and the witnesses received their first ivermectin' dose. The clinical tolerance of the treatment appears good and, even improved during the second dose one year after. Among the subjects treated in May 1987, 15.2% of them showed secondary reactions mostly discrete or moderate, precocious and quickly reversible after a second dose. Only 8 of them were incommodated in their daily occupations. A second treatment of these same subjects one year later, caused reactions of feeble intensity 3.7% only. The research of intolerance risk factors, incriminated the high density of microfilaremia. This incite to be careful in mass treatment of hyperendemic area.