Attention deficit hyperactivity disorder (ADHD) is a severe and often debilitating mental disorder, which begins in childhood and can persist into adulthood. Both major classificatory systems, ICD-10 and DSM-IV, include the age-of-onset criterion (AOC) requiring clinically relevant symptoms before the age of 7 years. In clinical practice, particularly in adult psychiatry, it is often difficult to establish this AOC when ADHD remained unrecognized in childhood. In the literature, there is controversy about the validity of this criterion. In order to explore the validity of the AOC, we performed a retrospective study in adult patients with ADHD. Fifty consecutively diagnosed patients were subjected to a standardized diagnostic procedure. Depending on the onset of clinical symptoms, patients were divided in an early onset and in a late onset ADHD subgroup. Fourteen patients (28%) described late onset ADHD. In four of these patients (8% of the total sample), this assessment was confirmed by parent ratings. There was no difference between early onset and late onset ADHD groups in terms of psychopathology or psychiatric comorbidity.