Attention deficit hyperactivity disorder (ADHD) is increasingly recognized as a legitimate adult diagnostic category. Yet the nature of and comorbidities and adaptive impairments associated with adult ADHD have received little scientific investigation. The present study, therefore, compared 172 adults diagnosed with ADHD with 30 adults referred to the same adult ADHD clinic who were not so diagnosed. The ADHD group showed a significantly greater prevalence of oppositional, conduct, and substance abuse disorders, and greater illegal substance use than control adults. Moreover, adults with ADHD displayed greater self-reported psychological maladjustment, more driving risks (speeding violations), and more frequent changes in employment. Significantly more ADHD adults had experienced a suspension of their driver license, had performed poorly, quit, or been fired from their job, and had a history of poorer educational performance and more frequent school disciplinary actions against them than adults without ADHD. Multiple marriages were more likely in the ADHD group as well. Contrary to previous studies, anxiety and mood disorders were not found to be more prevalent in the ADHD than in the control group. Results suggest that ADHD in adults is associated with relatively specific risks for disruptive behavior disorders, school and job performance problems, and driving risks.