Until quite recently, the literature on attention deficit hyperactivity disorder (ADHD) and hyperkinetic disorder (HKD) may have given the impression that they were solely problems of males, since study after study concentrated on boys only. Such a strategy may well be justified. There is little doubt that a male predominance is one of the consistent findings about ADHD and HKD and this must raise the possibility that affected females are in some ways different. The research approach of studying the sexes separately has much to be said for, but for a long time, the process tended to stop with the investigation of boys. However, in recent years there has been a considerable increase of interest in the question of sex differences in ADHD and HKD, both in terms of the practical concerns of clinicians treating girls and in terms of finding answers to questions about the aetiology of the disorder which could account for the sex differences. In particular, recent research has aimed at describing differences between affected males and affected females. In recent years a major advance in ADHD research has been the use of genetically informative strategies to illuminate the question of how psychopathology develops. It seems probable that this approach will in future provide more satisfactory answers to this question. However, at the moment different studies contradict each other on key questions and the reasons for these contradictions are not obvious. One particular problem in the methodology needs to be highlighted from the very beginning.