Abstract 1. 1) Earlier work has drawn attention to the possibility that intense infections plus another factor are required before vision is impaired in onchocerciasis. 2. 2) In the Bugoye region of Western Uganda intense onchocercal infections are found in the majority of the population and it is probable that all those who have reached early adult life are infected. The infection is principally transmitted by S. damnosum. 3. 3) Intensity of infection has become maximal in those of 30–39 years, and thereafter remains virtually unchanged for the next 20 years of life. 4. 4) Attention is drawn to the absence of pruritus in a significant proportion of patients with heavy onchocercal infections. 5. 5) Large inguinal or femoral lymphatic nodes, sometimes in pendulous sacks of skin, were encountered in eight of 67 patients. 6. 6) Ocular lesions including conjunctivitis, keratitis, iritis, and choroido-retinitis were encountered in some of the patients together with many non-onchocercal lesions causing impairment of vision. Anterior segment lesions were present in 29 patients, anterior and posterior segment lesions in 16, posterior segment lesions in eight and no ocular lesions in 14. 7. 7) Intensity of infection cannot be proved in this series to be associated with posterior segment lesions, but evidence was obtained suggesting that it may be so associated. 8. 8) No evidence in this study was found of vitamin A deficiency as a cause of choroidoretinitis in onchocerciasis. 9. 9) Evidence was also obtained that time, measured in decades rather than years, may be the principal factor which in addition to heavy onchocercal infection is required before impairment in visual acuity takes place.