Although Attention Deficit Hyperactivity Disorder (ADHD) is recognised to be a familial and heritable disorder, little is known about the broader family characteristics of having a parent with ADHD problems. The main aim of this study was to investigate the relationship between parent ADHD problems, child clinical presentation and family functioning in a sample of children with ADHD. The sample consisted of 570 children with ADHD. Child psychopathology was assessed using a semi-structured diagnostic interview. Questionnaires were used to assess ADHD in the parents (childhood and current symptoms), family environment and mother/father-child relationship. Parental ADHD problems were associated with a range of adverse clinical outcomes in children with no difference in effects for mothers with ADHD problems compared to fathers with ADHD problems. Levels of maternal hostility were higher in families where mothers had ADHD problems, but reduced where fathers had ADHD problems. Parental ADHD problems index higher risk for more severe clinical presentation of ADHD in children and higher levels of family conflict (where there are maternal but not paternal ADHD problems). This study highlights that children with more severe behavioural symptoms are more likely to have a parent with persistent ADHD which has important implications when considering treatment and intervention strategies.