Sleep disorders are common in adults with attention deficit/hyperactivity disorder (ADHD). Two sleep questionnaires; the Brown ADD Scale, the Symptom-Checklist-90-R, and a self-developed symptom questionnaire were administered in this study. In a sample of 61 patients without comorbidity, current substance abuse and medication intake, sleep problems were reported more often than in healthy controls. A lack of being refreshed in the morning was very closely associated with ADHD symptomatology whereas insomnia was related to the presence of comorbidity and depressive symptoms. In the total sample (N = 120), medication intake (mainly stimulants) was not related to any of the sleep parameters. It seems important to screen adult patients with ADHD for the presence of sleep disorders, especially insomnia, restless legs syndrome, and sleep-related breathing disorders (the last in conjunction with the body mass index). Since primary sleep disorders are associated with cognitive impairment, one might expect that ADHD symptomatology may improve if comorbid sleep disorders are adequately treated in addition to the specific ADHD treatment.