Attention-deficit/hyperactivity disorder (ADHD) is a disorder characterized by inattentive and/or hyperactive-impulsive behaviors that persist for 6 months or longer. About 6% of children and adolescents and about 3% of adults have ADHD. Because the symptoms are similar in children and adults, it has been assumed that ADHD is the same disorder regardless of age. Thus, most medical professionals have assumed that adults with ADHD have had ADHD since they were young and that many kids with ADHD become adults with ADHD. Despite this widely held belief, there have been few studies directly addressing this issue.