BACKGROUND AND AIM: Air pollution exposure has been shown to damage the brain but its associations with the development of psychopathology are not fully characterized. This study tested whether early life air pollution exposure is associated with greater psychopathology at the transition to adulthood. METHODS: Air pollution exposure and young adult psychopathology were investigated in the Environmental-Risk (E-Risk) Longitudinal Twin Study, a population-representative UK-based cohort of 2,232 children born in 1994-1995 and followed to age 18 years. High-resolution annualized estimates of outdoor nitrogen oxides (NOx) and fine particulate matter (PM2.5) linked to age-10 and age-18 home addresses were combined with data on mental-health disorder symptoms assessed through structured interview at age 18. Symptom counts were transformed through confirmatory factor analysis into continuous measures of General Psychopathology and Internalizing, Externalizing, and Thought Disorder symptoms, all standardized to mean(SD)=100(15). Adjustments were made for high-resolution measures of disadvantageous neighborhood characteristics, including socioeconomic deprivation, physical dilapidation, social disconnection, and dangerousness. RESULTS:After adjustment for family and individual factors, each interquartile range increment increase in NOx exposure was associated with a 1.40-point increase (95%CI=0.41-2.38, P=.005) in General Psychopathology. Associations between continuously measured PM2.5 and General Psychopathology were non-significant; however, those in the highest quartile of PM2.5 exposure scored 2.04-points higher (95%CI=0.36-3.72, P=.017) than those in the bottom three quartiles. Co-pollutant models including both NOx and PM2.5 implicated NOx as the significant factor. NOx exposure was associated with all secondary outcomes, although associations were weakest for Internalizing, medium for Externalizing, and strongest for Thought Disorder symptoms. Despite NOx concentrations being highest in neighborhoods with worse physical, social, and economic conditions, adjusting estimates for neighborhood characteristics did not change the results. CONCLUSIONS:Youths exposed to higher levels of NOx outdoor-air pollution experienced greater psychopathology at the transition to adulthood. Air pollution may be a non-specific risk factor for the development of mental illness. KEYWORDS: Air pollution, Oxides of nitrogen, Mental health outcomes, Children's environmental health