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Background: Maternal smoking during pregnancy has been consistently associated with disruptive behaviour in male offspring; however, results for girls are inconsistent and little is known about emergent patterns in young children. Additionally, it is unclear whether maternal smoking is independently associated in offspring with hyperactivity–inattention or only when it co-occurs with conduct problems. Further, few studies have controlled for a broad range of maternal psychosocial problems. Methods: Associations between self-reported smoking in pregnancy and maternal reports of externalising behaviour were analysed in more than 13 000 3-year-old boys and girls in the UK Millennium Cohort Study. Conduct and hyperactivity–inattention problems were assessed usin