The influence of breast feeding on developmental tests at five years of age was assessed in a national study of 13,135 children. A positive correlation was found between duration of breast feeding and performance in tests of vocabulary and visuomotor co-ordination, behaviour score, and measurements of height and head circumference. The effect on vocabulary and visuomotor co-ordination persisted when allowance was made for intervening social and biological variables, but that on height and head circumference disappeared. The breast-feeding/behaviour score association remained significant but became non-linear after adjustment. Breast feeding had no discernable effect on speech problems during the first five years. It is concluded that breast feeding may have an effect on children's development at five years: the effect is relatively small but resistant to attempts at statistical control.