The relationship between severe onchocercal eye lesions (iritis, sclerosing keratitis, optic atrophy and choroidoretinitis) and (i) the prevalence and intensity of microfilariae (mf) of Onchocerca volvulus in skin snips from the iliac crest and outer canthus, and (ii) the prevalence of mf in the cornea and anterior chamber of the eye, was studied in 1414 persons from forest and savanna villages and 312 attenders at eye clinics. Ecologically the savanna of Sierra Leone more closely resembles the forest than the dry Sudan-savanna areas of West Africa, and in persons aged 30 years or more the combined prevalence of anterior segment lesions (iritis and sclerosing keratitis) was higher in the forest villages (20.6%) than in the savanna (12.7%). The higher loads of mf found in the forest compared to savanna villages could explain these results. Prevalence rates for posterior segment lesions (optic atrophy and choroidoretinitis) were 28.1% and 22.6% in the forest and savanna respectively. Although in villages from both zones there was a close association between mf in the anterior chamber and optic atrophy, other associations between posterior segment lesions and mf were either not significant or weak. In contrast, there was a strong association between anterior segment lesions and mf in the eye and the concentration of mf at the outer canthus. This association was stronger for iritis than for sclerosing keratitis.