Sixty boys, aged from 6 to 10 years, were studied after their referral to psychiatric clinics for antisocial or disruptive behaviour. Their scores on reliable measures of hyperactivity, defiant behaviour, emotional disorder and attention deficit were taken for the home, school and clinic settings; and subjected to two techniques of cluster analysis. Both gave a similar set of clusters, one of which had high scores on all measures of hyperactivity and attention deficit. Membership of this cluster was associated with a lower IQ, a younger age of problem onset and referral, an abnormal neurological examination, a history of developmental delay, smaller family size, poor peer relationships and a high rate of accidental injuries; and it predicted a good response to stimulant medication in a controlled trial. Other research on the classification of hyperactivity is discussed, and proposals are made for the criteria of a rather narrow definition of 'hyperkinetic conduct disorder'.