The relationships between onchocercal infection, visual acuity and mortality have been examined using epidemiological data gathered by the Onchocerciasis Control Programme from 66 villages in West Africa. All of these villages were surveyed at least twice. 18,778 persons were registered at the first surveys which were conducted around the time when control activities started. The second surveys were conducted two to five years later. 14,961 persons (79.7%) were alive and registered again, 786 (4.2%) had died, 2776 (14.8%) had moved and 255 (1.4%) could not be traced. The prevalence of blindness and of visual damage other than blindness both increased markedly with age, and the degree of visual damage was strongly associated with level of microfilarial infection. Among adults, the prevalence of blindness was higher for males than for females at all ages. The prevalence and intensity of microfilarial infection were also higher among males but this was not sufficient to explain their excess blindness. Logistic regression analysis showed that males were 1.5 times more likely to be blind than females of the same age and same level of microfilarial infection. The presence of visual damage at the first survey considerably increased the risk of mortality between the first and second surveys for both males and females. Mortality rates were three to four times higher among the blind and about 1.5 times higher among those with visual damage other than blindness compared with those with no visual damage. There was some evidence, for males but not for females, that mortality was also directly related to microfilarial load.(ABSTRACT TRUNCATED AT 250 WORDS)