Background <p>Intervention programs can effectively reduce children’s aggressive behavior problems. However, surprisingly few studies have investigated through what underlying mechanisms these interventions exert their beneficial effects. Doing so requires high-frequency measurements of change mechanisms throughout the course of treatment. We used such a design to examine adaptive anger regulation and hostile intent attribution as mechanisms of change in an intervention to reduce children’s aggressive behavior problems.</p> Methods <p>In total, 76 boys with aggressive behavior problems (<i>M</i>_age = 10.58; 96.1% born in the Netherlands) participated in an 11-session cognitive behavior therapy intervention. During each treatment session, we used brief 3-item measures to assess children’s self-reported anger regulation, hostile intent attribution and aggression, as well as parent-reported child aggression.</p> Results <p>Results of latent growth curve model analyses showed that individual change in child-reported aggression was significantly associated with co-occurring individual change in adaptive anger regulation and hostile intent attribution. For parent-reported aggression, such associations were found for hostile intent attribution but not adaptive anger regulation.</p> Conclusions <p>These findings provide first indications for anger regulation and hostile intent attribution as within-intervention change mechanisms, suggesting that these mechanisms may be effective intervention targets to reduce children’s aggressive behavior problems.</p>