The Onchocerciasis Control Programme in West Africa has recently extended its operation in southern Benin, Ghana and Togo. To estimate the number of people infected and blinded by onchocerciasis and to describe the distribution and severity of the disease in the extension area, 99 villages were selected, using a stratified random sampling procedure, and surveyed. All the ecological and entomological information available was used in the sampling procedure and in the selection of 87 non-representative villages surveyed to confirm the findings. The study estimated that 590,468 people are infected and 11,715 blind from onchocerciasis out of a rural population of 1,878,234. The Pru, Asukawkaw and Mono river basins were areas with high risk of onchocercal blindness. The Oueme and Zou river basins in Benin and the mountainous areas between Ghana and Togo were classified as areas with medium risk of onchocercal blindness. The other parts of the study area presented low or no risk of onchocercal blindness. By detecting the river basins where villagers are at risk of onchocercal disease this study permits the selection of populations for disease control based on mass distribution of ivermectin, a microfilaricide.