Birth to Ten now Birth to Twenty (BT20), is the largest and longest running longitudinal birth cohort study in Africa. In this paper, the methods, magnitude and significance of recruitment, follow-up and attrition are described. Although more than 5000 births were notified in the area in the 7-week enrollment period in early 1990, only 3275 children were established to have been born to women who were residents in the Greater Johannesburg Metropolitan area for at least the first 6 months of the child's life. Seventy per cent of these children and their families have been followed up for more than 12 years, indicating an average attrition rate of less than 3% per annum, with most attrition occurring in the first 2 years of the study. The most common reason for attrition was movement out of the study area, although detailed follow-up, and the extent of contact re-established at later points, indicate very high levels of circular migration among women and young children between urban and rural areas, as well as very high levels of residential mobility within urban areas. There has been no differential loss of vulnerable families and children. African women living in Soweto are the most consistent participants in the study. A bias, by population group membership and residential area, was introduced in the recruitment phases of the project by the difficulty of enrolling the small proportion of people in the metropolis, largely Whites, who used private delivery services in 1989-90.