BACKGROUND: Although previous studies have reported negative associations between exposure to air pollution and cognition, studies of the effects of prenatal and postnatal exposures in early childhood have been limited. OBJECTIVES: We sought to assess the role exposure to line particulate matter (PM2.5) during clilferent prenatal and postnatal windows may play in children's cognitive development at school age. METHODS: Within the Brain Development and Air Pollution Ultrafine Particles in School Children (BREATHE) Project, we estimated residential PM2.5 exposures by land use regression for the prenatal period and first seven postnatal years of 2,221 children from Barcelona, Spain. The participants (8.5 +/- 0.9 y old) completed computerized tests assessing working memory, attentiveness, and conflict network during four visits in 2012-2013. We used linear mixed effects and distributed lag models to assess the period of exposure to PM2.5 in association with cognitive development. RESULTS: Inverse associations were identified between PM2.5 exposure during the fifth and sixth postnatal years and working memory, with boys showing much higher vulnerability. Regarding attention functions, exposure to higher PM2.5 levels during the prenatal period and from the fourth postnatal year were associated with a reduction in conflict network performance, though we found no association with attentiveness. The overall estimated cumulative effect of a 10 mu g 111-3 increase in PM2.5 resulted in a reduction in the working memory d score of 19.50 [95% confidence interval (CI): 31.44, 7.57] points and an increase in the conflict attentional network of 11.31 (95% CI: 6.05, 16.57) milliseconds, indicating a poorer performance. CONCLUSIONS: Early life exposure to PM2.5 was associated with a reduction in fundamental cognitive abilities, including working memory and conflict attentional network.