Multiple etiological factors (e.g., biological and personality predispositions) have been linked to the development of aggression. The purpose of the present study was to examine the relation between proactive/reactive aggression and biological (HPA‐axis functioning) and personality characteristics (subdimensions of psychopathy) among girls at risk for aggressive behavior. Participants included girls (N = 158) admitted for acute psychiatric inpatient treatment (M age = 9.72; SD = 1.99). Parents completed a measure of proactive/reactive aggression and psychopathy upon admission. Fasting plasma cortisol levels were obtained the morning following the child's admission. Correlational analyses revealed a significant negative correlation between cortisol and the narcissism and impulsivity subdimensions of psychopathy as well as proactive/reactive aggression. A significant positive relation between proactive and reactive aggression and the three subdimensions of psychopathy was also observed. Path analyses revealed that only narcissism was uniquely and positively related to proactive and reactive aggression. Tests of indirect effects from cortisol to aggression through subdimensions of psychopathy indicated significant pathways via narcissism to proactive and reactive aggression. The findings support previous research linking narcissism uniquely to aggression for girls and suggest that the relation between cortisol and proactive/reactive aggression is mediated by narcissism. Aggr. Behav. 40:263–272, 2014. © 2013 Wiley Periodicals, Inc.