BACKGROUND: Air pollution from traffic has been associated with cardiorespiratory diseases in children and adults, but there is little information on its potential neurotoxic effects. This study aimed to investigate the association between exposure to nitrogen dioxide (NO(2)), as a marker of traffic-related air pollution, and cognitive development in children. METHODS: A population-based birth cohort from southern Spain was followed from the age of 4 years for 1 year. Complete data for analyses were gathered on 210 children living in urban and rural areas. NO(2) exposure was predicted by means of land use regression models. A standardised version of the McCarthy Scales of Children's Abilities (MSCA) was used to assess children's motor and cognitive abilities. Multivariate analyses were performed to evaluate the relation between exposure to NO(2) and MSCA outcomes, adjusting for potential confounders. RESULTS: A negative effect of NO(2) was found across all MSCA subscales, despite low predicted NO(2) exposure levels (5-36 microg/m(3)). Children exposed to higher NO(2) (>24.75 microg/m(3)) showed a decrease of 4.19 points in the general cognitive score and decreases of 6.71, 7.37 and 8.61 points in quantitative, working memory and gross motor areas, respectively. However, except for gross motor function, associations were not statistically significant. CONCLUSION: Although results were not statistically significant, the associations found between exposure to NO(2) and cognitive functions suggest that traffic-related air pollution may have an adverse effect on neurodevelopment, especially early in life, even at low exposure levels.