Male and female residents on a Guatemalan coffee plantation where Onchocerca volvulus infections were hyperendemic were offered oral ivermectin (100-200 micrograms/kg) as part of a community-wide treatment programme for onchocerciasis. Forty-five persons were treated and then questioned daily for 28 d about changes in their health. Those with complaints were monitored until all signs and symptoms had resolved. Sixty-seven percent complained of some adverse event after treatment; 60% developed observable adverse reactions attributed clinically to ivermectin. No reaction was life-threatening; the most common were oedema (53%) and fever (47%). Expulsion of intestinal helminths was reported by 38%. Almost all reactions began 24-48 h after treatment; their mean duration was 5 d, despite treatment with acetaminophen and antihistamines. Three patients had oedematous changes lasting over 2 weeks. Incidence, but not severity, of reactions was related to the pretreatment density of microfilariae in skin.