Posterior segment ocular lesions were examined in 244 selected patients with onchocerciasis from the Sudan-savanna and rain-forest areas of the United Cameroon Republic. Fluorescein fundus angiography was performed on 210 cases. As a result of this study the following conclusions were drawn: 1. The heterogeneity of single lesions, and the occurrence of different types of lesion in a single eye or two eyes, prevented subdivision of the disease into distinct forms and suggested that inflammation alone was responsible for fundus lesions in onchocerciasis. 2. Optic nerve disease, alone or in the presence of choroido-retinal changes, was responsible for a large proportion of the blindness due to posterior segment lesions in onchocerciasis (87.6 per cent in this series).