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Families of children with attention-deficit/hyperactivity disorder (ADHD) report higher rates of conflict within the family and more negative parent–child relationships. This study aimed to test whether negative parent–child relationships have a risk effect on ADHD symptoms using two complementary designs.The first sample included 886 twin pairs, aged 11–17 years, derived from a population-based twin study. The second sample was derived from a longitudinal community study and included 282 parents and their children, aged 11–14 years. Questionnaires were used to assess ADHD symptoms and hostility in the mother–child and father–child relationship. Bivariate genetic analysis was used to test the contribution of genetic and environmental factors to the association between parent–child hostility and ADHD symptoms in the twin sample. Cross-lagged and reciprocal effects models were used to test for a bidirectional relationship between parent–child hostility and ADHD symptoms over time in the longitudinal study.For boys, both genetic and environmental factors contributed to the link between mother–son hostility and ADHD symptoms, but genetic factors alone explained the association between father–son hostility and ADHD symptoms. For girls, the association between ADHD symptoms and mother–daughter hostility as well as father–child hostility was attributed to genetic factors alone. The longitudinal study provided evidence of boys' ADHD symptoms impacting upon mother–son hostility both within and across time. There were no effects in the opposite direction.A causal hypothesis of family relations influencing ADHD symptoms was not supported. Boys' ADHD symptoms appear to have an environmentally mediated impact upon mother–son hostility.