Adolescents who witness interparental violence (IPV) are at increased risk for perpetrating aggressive acts. They are also at risk for post-traumatic stress disorder (PTSD). In this study, we examined the relation between exposure to maternal vs. paternal physical IPV and adolescent girls' and boys' aggressive behavior toward mothers, fathers, friends, and romantic partners. We also assessed the influence of PTSD (as assessed by the Diagnostic Interview for Children and Adolescents-IV (DICA-IV)) on the relation between exposure to IPV and aggressive behavior. Participants were 63 girls and 49 boys, ages 13–18, consecutively admitted to a youth correctional facility or assessment facility designated to serve aggressive and delinquent youth. Structural equation modeling was used to estimate unique relations between exposure to maternal vs. paternal IPV and youth aggression in relationships. Girls who observed their mothers' aggressive behavior toward partners were significantly more aggressive toward friends. Similarly, boys who witnessed their fathers' aggression were significantly more aggressive toward friends. Adolescent girls and boys who observed aggression by mothers toward partners reported significantly higher levels of aggression toward their romantic partners. Approximately one third of our sample met PTSD criteria; the relation between exposure to parental IPV and aggression was stronger for individuals who met criteria for PTSD. The implications of understanding the relations between parents' and their daughters' and sons' use of aggression are discussed within the context of providing support for families in breaking intergenerational patterns of violence and aggression. Aggr. Behav. 32:385–395, 2006. © 2006 Wiley-Liss, Inc.