Abstract
Recent population-based longitudinal studies concluded that most adults with attention deficit/hyperactivity disorder (ADHD) symptoms would not have a childhood history of ADHD, leading to the concept of adult-onset ADHD. In a large, well-characterized clinical population of 446 adults with a primary complaint of ADHD, we reported a low frequency of adult-onset ADHD (6.9%), being a primary isolated condition in 2.8%. They had less severe symptoms and tendencies for higher hypersomnolence disorder comorbidity than patients with typical childhood-onset ADHD. Our findings reinforce the requirement to exclude other disorders that might overlap with ADHD or mimic ADHD symptoms in adulthood onset patients.