General problems regarding diagnostic criteria and classification of attention deficit/hyperactivity disorder are discussed. A comparison of the most recent versions of the present diagnostic systems shows that they have converged and now overlap each other considerably. ICD-10 (clinical criteria) is still descriptive. DCR-10 (ICD-10's criteria for research) and DSM-IV both have operational criteria. The symptom lists of DCR-10 and DSM-IV are almost identical, and the criteria are very similar. The divisions into types, however, are very different, and none can be recommended. In clinical practice, and particularly in research, it is recommended that cases fulfill the criteria for "disturbance of activity and attention" (F90.0) in DCR-10 or "attention deficit/hyperactivity disorder, combined type" (314.01) in DSM-IV. There are considerable differences between former and present criteria. This must be considered when research results based on former criteria are examined.