Aggressive and violent behaviour is often associated with a diagnosis of attention-deficit hyperactivity disorder (ADHD). This article investigates the relationship between adult attention deficit disorder (ADD) and aggressive cognitions. The Aggression Questionnaire and the Brown Attention-Deficit Disorder Scales were administered to two samples: a “low-risk sample” comprising university students (n = 60), and a “high-risk sample” of prison inmates (n = 117). The prevalence of “probable ADD” was found to be higher in the prison sample (33%), compared with the university sample (20%). There were moderate correlations >r = .5 (p < .01) between total ADHD and Aggression Questionnaire scores in both groups. There were also moderate to strong correlations between subscales of both measures, including cognitive processes such as attention and memory and various forms of aggression, which were particularly apparent in the prison sample. The relationship between ADHD and aggressive behaviour suggests that the treatment of adult ADHD may aid in the management of aggressive behaviour.