Mass treatments with ivermectin have been undertaken each year since 1987 in an area hyperendemic for onchocerciasis in northern Cameroon. The impact of these successive treatments on the incidence of infection in humans was evaluated by comparing the prevalence of skin microfilariae (PMF) and the mean microfilarial skin densities (MFD) observed in 1987 and 1992 in 5-7-year-old children who had never taken the drug but who were members of the treated communities. In 1992, the PMF and the MFD in children in this age group who never received ivermectin were reduced by 55% and 77%, respectively, in comparison with the values observed in 1987, before the first treatment round. These results reflect a pronounced reduction in the intensity of the transmission of Onchocerca volvulus in the treatment zone. The influence of the ivermectin treatment coverage in the human population, as well as the vectorial capacity and the dispersal of the vector blackflies, on the transmission of onchocerciasis is discussed.