This study examined relations among family environment, cortisol response, and behavior in the context of a randomized controlled trial with 92 children (M = 48 months) at risk for antisocial behavior. Previously, researchers reported an intervention effect on cortisol response in anticipation of a social challenge. The current study examined whether changes in cortisol response were related to later child aggression. Among lower warmth families, the intervention effect on aggression was largely mediated by the intervention effect on cortisol response. Although the intervention also resulted in significant benefits on child engaging behavior, cortisol response did not mediate this effect. These findings demonstrate meaningful associations between cortisol response and aggression among children at familial risk for antisocial behavior.