Achenbach Child Behavior Checklist ratings were collected on boys aged 6 to 11 years screened as not having significant behavior problems, as clinic-referred boys, and as boys diagnosed as having Attention Deficit-Hyperactivity Disorder alone or together with Oppositional-Defiant Disorder or Conduct Disorder. Cluster analysis resulted in a classification system related to DSM-III-R classification of the boys and parent self-report of mildly antisocial acts. Clusters also enhanced the ability of the instrument to detect Attention Deficit Hyperactivity Disorder and disruptive behavior disorders.