A three-year placebo-controlled, double-blind trial involving 7148 persons was conducted to determine adverse reactions to ivermectin given annually for treatment of onchocerciasis by mass distribution. Musculoskeletal pains, oedema of the face or extremities, itching and papular rash were statistically associated with ivermectin treatment in the first year. In the second year of treatment these reactions were less frequently reported, although still more prevalent in ivermectin recipients. During the third year of treatment all persons received ivermectin. Persons who had missed ivermectin treatment in the second year did not have significantly more adverse reactions after treatment in the third year. Those who had received placebo during the first 2 years reported some adverse reactions more frequently following ivermectin than did those who had received ivermectin from the beginning of the study. Oedema was the adverse reaction of greatest concern to patients and this involved the face most commonly. A single episode of bullous skin lesions developed in 5 persons who had received ivermectin. These lesions did not reoccur with subsequent treatment. No episode of hypotension after treatment was observed.