Until recently, little was known about the epidemiology of attention-deficit/hyperactivity disorder (ADHD) in adults. Bottom-up studies following children with ADHD into adolescence had shown variable rates of persistence, some of which depended on the definitions used. The traditional diagnosis was complicated by the introduction of the Diagnostic and Statistical Manual of Mental Disorders, Third Edition, which stated that ADHD could be diagnosed with inattentive symptoms alone. This resulted in diagnostic inconsistency as earlier investigations demanded the presence of hyperactivity while others did not. Diagnosis also depended on the site, the cohort, whether interviews versus rating scales were employed, and whether the subject or their parent were the source of information.