STUDY OBJECTIVE: Prior studies have shown that children are particularly sensitive to air pollution. This study examined whether children of low socioeconomic status suffered greater exposure to outdoor nitrogen dioxide than more affluent ones, both at their place of residence and at school, in a country with widespread state intervention for social equity. DESIGN: Local scale data on outdoor nitrogen dioxide obtained from a validated air pollution model were analysed, along with all school children accurately geocoded to their building of residence and school. PARTICIPANTS: All 29,133 children in grades one through nine (aged 7 to 15 years) residing and attending school in Malmö, Sweden, in 2001. MAIN RESULTS: Defining the socioeconomic status of children according to the mean income in their residential building, the spatial scan statistic technique allowed the authors to identify eight statistically significant clusters of low socioeconomic status children, all of which were located in the most polluted areas of Malmö. Four clusters of high socioeconomic status children were found, all of them located in the least polluted areas. The neighbourhood socioeconomic status better predicted the nitrogen dioxide exposure of children than the socioeconomic status of their building of residence. Exposure to nitrogen dioxide at the place of residence and school of attendance regularly increased as the socioeconomic status of a child's neighbourhood of residence decreased. CONCLUSIONS: Evidence of environmental injustice was found, even in a country noted for its egalitarian welfare state. Enforcement of environmental regulations may be necessary to achieve a higher level of environmental equity.