Aggressive behavior between children in schools is a topic that receives much interest as violence and aggressive behavior cause many maladaptive social outcomes in the school setting. In the current study the Implicit Association Test (IAT) was adapted as a measure of children's implicit aggression, by assessing the association of the self category (vs. other) with the attribute concept of aggressive (vs. peaceful). In addition to the IAT, a questionnaire measure to assess explicit aggression was utilized. The main goal of the present contribution was to examine the predictive validity of measures of implicit as well as explicit aggression in predicting the scores in a competitive computer game as an indicator of children's aggressive behavior. Taken together, the results indicated that measures of implicit and explicit aggression could serve as reliable predictors of children's aggressive behavior and that the IAT possesses incremental validity in addition to the self-report measure. (Contains 2 notes.)