The prevalence, intensity and clinical manifestations of onchocerciasis were investigated in three village communities along the Bahr El Arab and its tributaries in Southern Darfur, Western Sudan. Onchocerca volvulus has not been reported from this region before. Over 300 people were examined and the selection of patients was aimed at obtaining a cross-sectional view of the disease at all ages and in both sexes. Prevalence rates were high (67.5%, 28.6% and 32% in Titribi, Radom and Kafia Kingi, respectively). The intensity of infection in young adults was generally about 30 mf/mg, but ranged up to 100 mf/mg. Infections were detected in subjects as young as two years old; about one quarter of those sampled in Titribi had nodules, mostly in the pelvic region. Clinical signs of acute and chronic dermal changes were especially marked in Titribi. This village was located closest to the breeding sites, which appear in the rainy season only. More than a third of those samples had severe pruritus and showed many self-inflicted excoriations. Both anterior and posterior eye segment changes were detected in each community, and cases of onchocercal blindness were attributed to sclerosing keratitis and to optic and chorioretinal atrophy. One case typical of intensely localized disease was seen, where the affliction was unilateral and severe with oedema and pigment changes, but very few microfilariae present. Onchocerciasis appears to be well established in this region and has apparently caused abandonment of some settlements in recent years.(ABSTRACT TRUNCATED AT 250 WORDS)