This study employed a playroom observation technique to examine the behavioral differences between hyperactive and aggressive boys. Subjects were clinic-referred boys assigned to Hyperactive, Aggressive, or Hyperactive plus Aggressive groups on the basis of behavior checklists and ratings of psychiatric chart information. While significant discrimination was obtained in all three settings (free play, restricted play, and restricted academic), behavioral differences among the subject groups were most pronounced during the restricted academic period. Discriminant function analysis for the restricted academic period resulted in accurate classification of 86% of the subjects as hyperactive, aggressive, or hyperactive plus aggressive. The present findings suggest that children with externalizing disorders can be distinguished in light of their observed clinic behavior in the restricted academic setting.