Background: 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. Method: 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. Results: 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. Conclusions: 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.