We comprehensively reviewed research assessing differences in attention-deficit hyperactivity disorder (ADHD) subtypes to examine the possibility that ADHD/ combined type (ADHD/Q and ADHD/predominantly inattentive type (ADHD/I) are distinct and unrelated disorders. Differences among subtypes were examined along dimensions identified as being important in documenting the distinctiveness of two disorders. These include essential and associated features, demographics, measures of cognitive and neuropsychological functioning, family history, treatment response, and prognosis. Important differences among subtypes were found in several areas of study, supporting the conclusion that ADHD/C and ADHD/I may best be characterized as distinct disorders. We identify major limitations of the available research and present future directions for research.