Epidemiological data gathered by the Onchocerciasis Control Programme from 66 villages in West Africa have been analysed. All of these villages were surveyed twice and 10 a third time. The first surveys were conducted around the time when control activities started and provide baseline data against which to assess the effectiveness of control. The second and third surveys were conducted two to five years later. 18,778 persons were registered in the first surveys and 15,783 (84.1%) were examined for microfilariae. The prevalence of microfilarial infection was higher among males, increased rapidly with age in both sexes until about 25 years and levelled off at about 90% in males and 85% in females. The age-sex adjusted prevalence rates in different villages varied from 5% to 93% but were above 60% in three quarters of the villages. After two to five years of control there was an over-all fall in the prevalence of infection of about 4 to 5% a year. Among children born since control started, only seven of 1871 examined were found to be infected with microfilariae compared to an expected number of 79, had there been no change in prevalence rates.