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BACKGROUND: Traffic-related air pollution is emerging as a risk factor for Alzheimer's disease (AD) and impaired brain development. Individual differences in vulnerability to air pollution may involve the e4 allele of Apolipoprotein E (APOE) gene, the primary genetic risk factor for AD. OBJECTIVE: We analyzed whether the association between traffic air pollution and neurodevelopmental outcomes is modified by APOE e4 status in children. METHODS: Data on parent-reported behavior problems (total difficulties scores, Strengths and Difficulties Questionnaire), teacher-reported attention-deficit hyperactivity disorder (ADHD) symptom scores, cognitive performance trajectories (computerized tests of inattentiveness and working memory repeated 2–4 times during January 2012–March 2013), and APOE genotypes were obtained for 1,667 children age 7–11 y attending 39 schools in or near Barcelona. Basal ganglia volume (putamen, caudate, and globus pallidum) was measured in 163 of the children by MRI (October 2012–April 2014.) Average annual outdoor polycyclic aromatic hydrocarbons (PAHs), elemental carbon (EC), and nitrogen dioxide (NO ) concentrations were estimated based on measurements at each school (two 1-wk campaigns conducted 6 months apart in 2012). RESULTS: APOE e4 allele carriers had significantly higher behavior problem scores than noncarriers, and adverse associations with PAHs and NO were stronger or limited to e4 carriers for behavior problem scores (P-interaction 0.03 and 0.04), caudate volume (P-interaction 0.04 and 0.03), and inattentiveness trajectories (P-interaction 0.15 and 0.08, respectively). Patterns of associations with the same outcomes were similar for EC. CONCLUSION: PAHs, EC, and NO were associated with higher behavior problem scores, smaller reductions in inattentiveness over time, and smaller caudate volume in APOE e4 allele carriers in our study population, and corresponding associations were weak or absent among e4 noncarriers. These findings support