An evaluation of the effects on onchocerciasis of a 3-year period of vector control was undertaken during 1978 in the Onchocerciasis Control Programme area in West Africa. The results revealed that the overall prevalence of ocular onchocerciasis showed only a slight decrease at the follow-up in 1978, but that there was significantly less infection among children in the age group 5-14 years as compared with 1975. There was a total incidence of ocular signs of onchocerciasis of 8.6% over the 3 years, but also a disappearance of those signs in 11.7% of the sample examined. The incidence of severe onchocercal eye manifestations was low, compared with similar areas of uncontrolled transmission. The particularly low incidence of sclerosing keratitis may be associated with the finding of a significantly decreased microfilarial load in the cornea, whereas the number of living microfilariae in the anterior chamber of the eye was apparently unchanged. The incidence of blindness due to onchocerciasis was low and confined to individuals who already presented severe eye manifestations of the disease before the beginning of the vector control campaign.