Aim: The aim of this article was to explore the effect of duration of breastfeeding on neurocognitive development. Method: The long-term effect of breastfeeding on neurodevelopment was examined through a battery of neuropsychological tests in 1403 children (693 females, 710 males; mean age 11y 9mo [SD 6mo], range: 10y 3mo-12y 8mo) who were originally recruited at 6 to 12 weeks of age for a clinical trial on acellular pertussis vaccines. An estimated IQ was obtained from scores of the vocabulary, similarities, block design, and coding tests. Breastfeeding data had been prospectively collected throughout the first year of life. Duration of exclusive breastfeeding was defined as the time during which children received breast milk without receiving any supplemental formula or food. Children were assessed at 10 to 12 years of age. We adjusted the analysis on test scores for multiple potential confounders. Results: Multivariate analysis showed a significant association between exclusive breastfeeding duration and test scores in the vocabulary (odds ratio [OR] 0.05; confidence interval [CI] 0.00-0.10; p=0.04) and similarities (OR 0.06; CI 0.01-0.11; p=0.03) tests. These associations have a negligible effect size, however. Scores on one writing praxis test subcategory decreased with increasing duration of both exclusive breastfeeding (OR -0.06; CI -0.11 to -0.01; p=0.03) and breastfeeding irrespective of consumption of other foods (OR -0.06; CI -0.11 to -0.01; p=0.03). A negative association was also found between one subcategory of the California verbal learning test and breastfeeding duration longer than 6 months (OR -0.21; CI -0.42 to -0.01; p=0.04). Interpretation: Breastfed healthy children may perform better on neuropsychological tests in the language domain at 10 to 12 years of age. However, the effect of breast milk on neuropsychological performance in healthy children may have a limited clinical relevance and is confounded by parental education. (Contains 3 tables.)