Attention-deficit hyperactivity disorder (ADHD) is a pervasive neurodevelopmental disorder characterized by symptoms of inattention and hyperactivity. In up to 50% of cases, children with ADHD have additional symptoms of emotional dysregulation including anger, irritability, or aggression. Aggression confers considerable additional risk for adverse health outcomes like illicit drug use and suicide. Aggression is associated with changes in cortisol, a steroid hormone involved in the stress response. The literature on the direction of the association is inconsistent and must be better characterized. Dysregulated cortisol may represent a treatment target in children with ADHD and symptoms of aggression. We will assess the association between urinary cortisol and the severity of aggression in children who have ADHD plus symptoms of emotional dysregulation.