Objective: The authors describe personality development and disorders in relation to symptoms of attention deficit hyperactivity disorder (ADHD) and autism spectrum disorders. Method: Consecutive adults referred for neuropsychiatric investigation (N=240) were assessed for current and lifetime ADHD and autism spectrum disorders and completed the Temperament and Character Inventory. In a subgroup of subjects (N=174), presence of axis II personality disorders was also assessed with the Structured Clinical Interview for DSM-IV Personality Disorders (SCID-II). Results: Patients with ADHD reported high novelty seeking and high harm avoidance. Patients with autism spectrum disorders reported low novelty seeking, low reward dependence, and high harm avoidance. Character scores (self-directedness and cooperativeness) were extremely low among subjects with neuropsychiatric disorders, indicating a high overall prevalence of personality disorders, which was confirmed with the SCID-II. Cluster B personality disorders were more common in subjects with ADHD, while cluster A and C disorders were more common in those with autism spectrum disorders. The overlap between DSM-IV personality disorder categories was high, and they seem less clinically useful in this context. Conclusions: ADHD and autism spectrum disorders are associated with specific temperament configurations and an increased risk of personality disorders and deficits in character maturation.