The authors tested whether emerging borderline personality disorder (BPD) symptoms mediated the association between childhood physical abuse (CPA) and aggression among incarcerated girls. Participants were 121 incarcerated adolescent girls (13–19 years old). Three forms of aggression (relational, overt, and violent offending behavior) and exposure to CPA by a parental figure were assessed using self-report inventories, whereas BPD symptoms were evaluated using a structured interview. Mediation models, including tests of indirect effects, were conducted in which each form of aggression was predicted from CPA with BPD symptoms entered as a mediator. A divergent pattern emerged in which BPD symptoms mediated the relationship between CPA and violent offending, but not less severe forms of overt aggression. Relational aggression, although correlated with CPA, was not associated with BPD symptoms. Implications for the conceptualization and treatment of girls' aggression within the context of interpersonal functioning are discussed.