Despite growing public resistance to the practice of female genital cutting (FGC), documentation of its prevalence, social correlates or trends in practice are extremely limited, and most available data are based on self-reporting. In three antenatal and three family planning clinics in South-west Nigeria we studied the prevalence, social determinants, and validity of self-reporting for FGC among 1709 women. Women were interviewed on social and demographic history, and whether or not they had undergone FGC. Interviews were followed by clinical examination to affirm the occurrence and extent of circumcision. In total, 45.9% had undergone some form of cutting. Based on WHO classifications by type, 32.6% had Type I cuts, 11.5% Type II, and 1.9% Type III or IV. Self-reported FGC status was valid in 79% of women; 14% were unsure of their status, and 7% reported their status incorrectly. Women are more likely to be unsure of their status if they were not cut, or come from social groups with a lower prevalence of cutting. Ethnicity was the most significant social predictor of FGC, followed by age, religious affiliation and education. Prevalence of FGC was highest among the Bini and Urhobo, among those with the least education, and particularly high among adherents to Pentecostal churches; this was independent of related social factors. There is evidence of a steady and steep secular decline in the prevalence of FGC in this region over the past 25 years, with age-specific prevalence rates of 75.4% among women aged 45-49 years, 48.6% among 30-34-year olds, and 14.5% among girls aged 15-19. Despite wide disparities in FGC prevalence across ethnic, religious and educational groups, the secular decline is evident among all social subgroups.