Recent research into attention deficit hyperactivity disorder (ADHD) raises many questions, one of which is: how do we define the subtypes and subthreshold groups of the condition? ADHD is too complex and heterogeneous a condition to assign all cases to two categories (‘ADHD’ or ‘non-ADHD’). However, no useful splitting is possible for all assumed subtypes of similar (but not identical) character, for either clinical practice or taxonomic research.