Thirty-one patients with onchocerciasis were treated with a single dose of 150-200 micrograms/kg ivermectin. They were observed for three days to detect acute reactions and were reassessed clinically and parasitologically three, six and twelve months after treatment. 22 patients were expatriates. All had early, light and often localised infections and were not reexposed to infection during the course of the study. 25 patients completed follow-up. Patients who relapsed were retreated with the same dose after an interval of not less than five months. Adverse reactions were less severe than those experienced by similar patients treated with diethylcarbamazine and only one patient was given corticosteroids. 17 patients (68%) relapsed within one year and two thirds of these had done so within 6 months. A similar pattern was seen after the second dose. A single dose of ivermectin, repeated every three to six months as necessary, is likely to be the treatment of choice for patients in non-endemic areas, lightly infected with Onchocerca volvulus, until a safe macrofilaricidal drug emerges. One third of such patients may be cured with each treatment.