According to the 1991/92 Tanzania Demographic and Health Survey, a Tanzanian woman has, on average, 6·1 births before she reaches age 50, a decline of about one birth per woman since the early 1980s. The major proximate determinant of fertility is universal and prolonged breastfeeding. An analysis of the social and demographic correlates of fertility shows that infant and child mortality, level of education and age at first marriage are among the factors which significantly influence fertility in Tanzania.