ADHD and comorbidity in childhood.

In recent years, evidence has been accumulating regarding high levels of comorbidity between attention-deficit/hyperactivity disorder (ADHD) and a number of disorders, including mood and anxiety disorders and conduct disorder. Thus, ADHD is most likely a group of conditions, rather than a single homogeneous clinical entity, with potentially different etiologic and modifying risk factors and different outcomes. Follow-up studies of children with ADHD indicate that subgroups of subjects with ADHD and comorbid disorders have a poorer outcome as evidenced by significantly greater social, emotional, and psychological difficulties. Investigation of these issues should help to clarify the etiology, course, and outcome of ADHD.