Adolescent antisocial behavior is a significant problem. Aggression Replacement Training (ART) is a multicomponent intervention developed to treat adolescent antisocial behavior. However, to date, no research has focused on the extent to which ART affects the cycles of antisocial behavior among youth in short-term residential programs. This article describes and reports results of a study investigating the relationship between ART and periodicities in antisocial behavior in a short-term shelter for adolescents. Time series data (519 days) on the antisocial behavior of 522 adolescents are analyzed using two methods: spectral analysis and harmonic regression analysis.