In this study, we explored the clinical characteristics, impairment and comorbidity in a sample of 45 adult patients with attention-deficit/hyperactivity disorder (ADHD). The collection of data is based on a naturalistic, retrospective approach using medical records documenting a comprehensive assessment of the patients. The sample was severely impaired in terms of academic achievement, employment and criminality, and had very high levels of comorbidity, especially alcohol and drug abuse, antisocial personality disorder and depression. Despite a high degree of contact with child psychiatric services in childhood, very few were diagnosed with ADHD, and many had a long period of psychiatric treatment as adults before the ADHD diagnosis was made. ADHD is in this sample of adults associated with severe impairment and comorbidity, and the connection between impairment and lack of proper diagnosis and treatment is discussed.