In a prospective study to determine the tolerance for and safety of ivermectin therapy for onchocerciasis in a hyperendemic area in Sierra Leone, 28 (32%) of 87 patients had adverse reactions that required treatment with acetylsalicylic acid and antihistamines, but none of the observed adverse reactions were considered life-threatening. A significant relation was found between the frequency and severity of side-effects and the degree of parasite infestation, as quantified by the skin-snip counts. Free administration of ivermectin to severely infected onchocerciasis patients is not recommended without some form of medical supervision.