The effects of breastfeeding on cognitive, visuomotor and language development were examined in healthy children born at full term, after they had reached 56 months of age. Three hundred and sixty-three children were breastfed for less than 5 months, and 363 for 5 months or more. The groups were matched pairwise having regard to maternal education and sex of the child. Significant differences were found in relation to scores reflecting general cognitive capacity, and the results of the visuomotor integration test between children breastfed for less than 5 months and those breastfed for 5 months or more, and between children of mothers who had smoked during pregnancy and non-smoking mothers. In multiple linear regression analysis prolonged breastfeeding was significantly related to scores reflecting general cognitive capacity and results of the visuomotor integration test. However, smoking by mothers during pregnancy was not significantly related to scores in cognitive tests. Biological factors, and factors such as lifestyle and social background, may be more important determinants of a child's development than breastfeeding.