BACKGROUND: The outcomes of attention deficit hyperactivity disorder (ADHD) have been studied extensively in the first decades of life, but less is known about ADHD in adulthood. Hence we investigated cross-sectional age-related differences in behavioural symptoms, neuropsychological function and severity of co-morbid disorders within a clinically referred adult ADHD population.MethodWe subdivided 439 referrals of individuals with ADHD (aged 16-50 years) into four groups based on decade of life and matched for childhood ADHD severity. We compared the groups on measures of self- and informant-rated current behavioural ADHD symptoms, neuropsychological performance, and self-rated co-morbid mood and anxiety symptoms. RESULTS: There was a significant age-related reduction in the severity of all ADHD symptoms based on informant-ratings. In contrast, according to self-ratings, inattentive symptoms increased with age. Neuropsychological function improved across age groups on measures of selective attention and response inhibition. There was a mild correlation between the severity of depression symptoms and increasing age. CONCLUSIONS: This observational study suggests that, in adulthood, ADHD symptoms as measured using informant-ratings and neuropsychological measures continue to improve with increasing age. However the subjective experience of people with ADHD is that their symptoms worsen. This dichotomy may be partially explained by the presence of co-morbid affective symptoms. The main limitation of the study is that it is cross-sectional rather than longitudinal, and the latter design would provide more conclusive evidence regarding age-related changes in an adult ADHD population.