One hundred and ninety eight patients with moderate to heavy infection with Onchocerca volvulus and with eye involvement in most, were allocated randomly to treatment with 100, 150 or 200 mcg/kg body weight of ivermectin or placebo given as a single oral dose in a double-blind dose finding study. The patients were drawn from an area under over ten years of vector control in Northern Ghana by the Onchocerciasis Control Programme, OCP. They underwent detailed clinical, laboratory and ophthalmological examination before treatment and in the review period of one year in hospital. Ivermectin given in a dose of 100, 150 or 200 mcg/kg eliminated microfilariae similarly slowly over 3-6 months and was associated with inflammatory reaction in the anterior segment which resolved without treatment. No changes in the fundus of the eye was detected by fluorescein angiography and no no-table other adverse eye reaction was observed. The ceiling of therapeutic activity of ivermectin in the eye is therefore put at 100 mcg/kg which is lower than the level fo 150 mcg/kg found in the skin. The apparent discrepancy may be due to different dose requirements on account of different mechanisms of action of ivermectin at the two sites. In the skin there is active killing while in the eye it is presumed there is a passive elimination of microfilariae.