The distinction between hyperactivity and conduct disorder was explored in a mixed group of 64 children referred to psychiatric clinics because of antisocial or disruptive behaviour. A semi-structured interview measure (the Parental Account of Children's Symptoms, PACS) proved to have adequate inter-rater reliability, internal consistency and factorial validity. The PACS scales of defiance and hyperactivity, and similar subscales from Conners' Teacher Rating Scale, were tested against laboratory and clinical measures of activity, attention, cognitive performance, psychosocial background and family relationships. The hyperactivity (but not the defiance) scales were associated with greater activity, younger age, poorer cognitive performance and abnormalities on a developmental neurological examination. The defiance (but not the hyperactivity) scales were associated with impairment of family relationships and adverse social factors. It was concluded that a dimension of inattentive, restless activity should be separated from one of antisocial, defiant conduct in children with psychiatric disorder.