ADHD combined type and ADHD predominantly inattentive type are distinct and unrelated disorders

We comprehensively reviewed research assessing differences in attention-def icit hyperactivity disorder (ADHD) subtypes to examine the possibility that ADHD/combined type (ADHD/C) and ADHD/predominantly inattentive type (ADHD/ I) are distinct and unrelated disorders. Differences among subtypes were ex amined along dimensions identified as being important in documenting the di stinctiveness of two disorders. These include essential and associated feat ures, demographics, measures of cognitive and neuropsychological functionin g, family history, treatment response, and prognosis. Important differences among subtypes were found in several areas of study, supporting the conclu sion that ADHD/C and ADHD/I may best be characterized as distinct disorders . We identify major limitations of the available research and present futur e directions for research.