The relationship between breast-feeding practices and childhood intelligence and language development at ages 3, 5 and 7 years was examined in a birth cohort of New Zealand children. The results showed that even when a number of control factors including maternal intelligence, maternal education, maternal training in child rearing, childhood experiences, family socio-economic status, birth weight and gestational age were taken into account, there was a tendency for breast-fed children to have slightly higher test scores than bottle-fed infants. On average, breast-fed children scored approximately two points higher on scales with a standard deviation of 10 than bottle-fed infants when all control factors were taken into account. It was concluded that breast-feeding may be associated with very small improvements in intelligence and language development or, alternatively, that the differences may have been due to the effects of other confounding factors not entered into the analysis.