The present study aimed at identifying the full range of mental disorders comorbid to attention-deficit/hyperactivity disorder (ADHD) in children and adolescents (age 4-17) diagnosed in Danish psychiatric hospitals between 1995 and 2010. A total of 14,825 patients were included in the study and comorbid disorders diagnosed concurrent with ADHD were identified. Associations of comorbid disorders with sex, age, and other mental disorders were investigated by logistic regression analysis. In the total sample, 52.0 % of the patients had at least one psychiatric disorder comorbid to ADHD and 26.2 % had two or more comorbid disorders. The most frequent comorbid disorders were disorders of conduct (16.5 %), specific developmental disorders of language, learning and motor development (15.4 %), autism spectrum disorders (12.4 %), and intellectual disability (7.9 %). Male sex was generally associated with an increased risk for neuropsychiatric disorders while female sex was associated more frequently with internalizing disorders. The analysis of associations between the various comorbid disorders identified several clusters highlighting the differential developmental trajectories seen in patients with ADHD. The study provides evidence that comorbidity with mental disorders is developmentally sensitive. Furthermore, the study shows that particular attention should be given to patients with neurodevelopmental disorders such as autism and intellectual disability in future longitudinal analyses. These disorders are very frequent in patients with ADHD, and the affected patients might follow a different course than patients without these disorders.