A community trial of the microfilaricide ivermectin undertaken in an hyperendemic focus of blinding onchocerciasis in Ghana. One objective was to investigate the safety of this new drug when used in mass treatments. 14,911 persons (61.5% of the census population) were treated with ivermectin. Of these, 15% reported with adverse reactions which were generally similar to those reported in the clinical trials. However, cutaneous reactions were relatively less frequent while brawny oedema of the limbs and inguinal gland pain were important. The severe reactions consisted of 37 cases of Severe Symptomatic Postural Hypotension (SSPH), 13 cases of severe fever and two cases of severe dyspnoea. The latter two cases represented life threatening situations, but there was no evidence that they were complications of ivermectin treatment. Only four of the SSPH cases required treatment. All severe adverse reactions were managed successfully and recovered within one day, usually within a few hours. The incidence of adverse reactions was highest the first day after treatment. Thirteen cases of delayed reactions were reported during a four-week follow-up. There was a highly significant relationship between incidence of adverse reactions and intensity of infection but no relation with ivermectin dosage within the range of 130-200 mcg/kg. The results suggest that ivermectin is sufficiently safe to be used in mass treatments. However, mass distribution of this drug should not be undertaken without adequate monitoring.