The extent to which ivermectin treatments have an impact on onchocerciasis transmission is a matter of some concern. We investigated this issue in the Mbam valley, a hyperendemic focus located in a forest-savannah mosaic area of Cameroon. Parasitological examinations of 5-9-year-old children, who had never received any antifilarial drug, were conducted before the first distribution of ivermectin in 1991-1993 and again in 2002, after four annual rounds of mass treatments. After matching for gender, age and village of residence, the prevalence and intensity of microfilaridermia corresponded respectively, in 2002, to 66.2 and 42.0% of the initial values. The decrease was more marked among the youngest children who, compared with the older ones, were submitted to the reduced force-of-infection earlier in their life. The results of the present study suggest that the specific vectorial competence of Simulium squamosum cytotype B, the vector of Onchocerca volvulus in the Mbam valley, allows a significant decrease in onchocerciasis transmission after several years of treatment, despite low therapeutic coverage. Though these results are encouraging, efforts should be made to improve the therapeutic coverage in the area.