- This study investigated the influence of a child’s DRD4 risk, parental levels of ADHD symptoms, and the interactive influence of these factors on the development of preschool aggression. Additionally, the study investigated the role of home chaos as a mediator between parental ADHD symptoms and child aggression. The sample consisted of 84 4.5-year-old children and their parents. Children were genotyped for the DRD4 polymorphism. ADHD symptoms were self-reported by parents when the child was 2 to 6 months old. Parental reports of home chaos and the child’s aggression were collected 4 years later. Child’s DRD4 risk and parental ADHD symptoms significantly contributed to the prediction of preschool aggression. However, contrary to our hypotheses, no interactions were found between the child’s DRD4 risk and the levels of parental ADHD symptoms. Home chaos played a mediating role in the relation between paternal ADHD symptoms and the child’s aggression. The relation between maternal ADHD symptoms and the child’s aggression was not significantly mediated through the level of home chaos. The current study emphasizes the importance of longitudinally investigating the contribution of parental ADHD symptoms to child aggression, while also exploring the differential contribution of maternal/paternal inattention and hyperactivity-impulsivity symptoms. Moreover, home chaos was found to be a significant environmental mechanism through which paternal ADHD symptoms affect children’s aggression in the preschool years.