Onchocerciasis, or river blindness, results from infection with Onchocerca volvulus. The parasite is endemic to West Africa, in both rain forest and savanna bioclimes. Several lines of evidence suggest that different strains of the parasite exist in the rain forest and savanna. Furthermore, epidemiologic evidence indicates that ocular onchocerciasis is most severe in savanna regions. This has led to the hypothesis that there is a strain association with ocular pathology. To test this hypothesis, parasites from villages in which severe and mild onchocerciasis were endemic were classified with two strain-specific DNA probes. A strong correlation (P less than .001) was found between disease severity and probe recognition, supporting the hypothesis that pathogenicity is strain related. The results suggest that pFS-1 and pSS-1BT may be used to predict the pathogenic potential of parasite populations throughout much of West Africa.