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Numerous ubiquitous environmental chemicals are established or suspected neurotoxicants, and infants are exposed to a mixture of these during the critical period of brain maturation. Evidence for associations with the risk of attention-deficit/hyperactivity disorder (ADHD) is sparse. We investigated measured perinatal and estimated postnatal chemical exposure levels in relation to ADHD.We used a birth cohort of Norwegian mother–child pairs enrolled 2005–2009 (HUMIS-NoMIC). In a subset of 1199 oversampled for neurodevelopmental outcomes, 27 persistent organic pollutants were measured in maternal breastmilk samples (14 PCBs, 5 organochlorine pesticides, 6 brominated flame retardants, and 2 perfluoroalkyl substances). We modelled pre- and postnatal exposures using a pharmacokinetic model. ADHD (n=40) was identified based on an ICD-10 diagnosis of hyperkinetic disorder in the national patient registry by 2014 (median age of 10.3 years). To identify associations and adjust for co-exposure confounding, we used elastic net penalised logistic regression models, and then used multivariable logistic regression models to obtain effect estimates for the selected exposures.Perfluorooctane sulfonate (PFOS) and β-hexachlorocyclohexane (β-HCH) were associated with increased odds and hexachlorobenzene (HCB) with decreased odds of ADHD diagnosis [confounder-adjusted odds ratio (OR) per interquartile range increase in breastmilk levels: 2.04 for PFOS; OR=1.64 for β-HCH; OR=0.36 for HCB]. Postnatal exposures showed similar results, whereas effect estimates for other chemicals were imprecise.In a multi-pollutant analysis of four classes of chemicals, early-life exposure to several persistent organic pollutants was associated with ADHD.