--- - |2+ Background Attention‐deficit/hyperactivity disorder (ADHD) is currently defined as a disorder with onset during childhood. Although ADHD occurs in adults as well as children, recent debate has focused on whether adult ADHD represents a continuation of a child‐onset disorder or if ADHD may, in at least some cases, have an adult onset. We therefore aimed to test the hypothesis of adult‐onset ADHD using a sample born relatively recently (1992–1999) in order to minimize confounding by secular changes in diagnostic practices. Methods We identified 74 individuals with a community diagnosis of ADHD first assigned during adulthood. We also identified individuals with childhood (N = 194) and adolescent (N = 394) community diagnoses of ADHD. These groups were compared with a comparison group (N = 14,474) on their childhood ADHD and neuropsychiatric symptoms, and rate of other psychiatric diagnoses during childhood. Results Having an adulthood community diagnosis of ADHD was associated with a mean increase in childhood ADHD symptoms of approximately three times that of the comparison group. Individuals with an adult community diagnosis of ADHD also displayed more autistic traits, motor problems, learning difficulties, tics, and oppositional behavior. Forty two percent of these individuals, compared with 1% of comparison cases, had a psychiatric diagnosis other than ADHD as children. In post‐hoc analyses of 21 ADHD cases showing few or no ADHD symptoms in childhood, we were unable to detect any other childhood symptomatology in only nine cases, of whom six were female. Conclusions Our results indicate that alternative explanations for data that appear to show adult onset ADHD, such as sex biases in diagnostic practices, need rigorous testing before adult onset ADHD can be accepted as a valid clinical construct. - Journal of Child Psychology and Psychiatry, EarlyView.