Attention-deficit/hyperactivity disorder (ADHD) is conventionally considered a children's disorder in which the symptoms naturally disappear with age. However, in reality, the functional remission rate is about 10% even in adulthood, and we have come to understand that 75% of people with ADHD in childhood experience a continuation of symptoms through adolescence. Epidemiological studies have reported a global prevalence of 3.4% for adult ADHD. The central distinctive feature of adult ADHD is attention deficit, while hyperactivity and impulsivity weaken or manifest differently over time. Additionally, since symptoms continue from childhood, the characteristics of ADHD become part of the personality of the individual, making it difficult to think of traits as symptoms. Therefore, with adult ADHD and its traits in mind, diagnosis should be made with due care and attention, especially as some of the symptoms of ADHD can also be seen in a number of other mental illnesses. Caution should be exercised in the differential diagnosis of ADHD in order to avoid over-diagnosis.