The purpose of this study was to examine the association between hypothalamic-pituitary-adrenal axis (HPA-axis) reactivity and proactive and reactive aggression in pre-pubertal children. After a 30-min controlled base line period, 73 7-year-old children (40 males and 33 females) were randomly assigned to one of two experimental tasks designed to elicit fear (N = 33) or frustration (N = 32), or a validity check condition (N = 8). This was followed by a 60-min controlled regulation phase. A total of 17 saliva samples for cortisol analysis were collected including 12 post-stress samples at 5-min intervals. Reactive and proactive aggression levels were assessed via the teacher-completed Aggression Behavior Teacher Checklist (Dodge and Coie, J Pers Soc Psychol, 53(6), 1146-1158, "1987"). Reactive aggression significantly predicted total and peak post-stress cortisol regardless of stress modality. Proactive aggression was not a predictor of any cortisol index. Examination of pure reactive, proactive, combined, or non-aggressive children indicated that reactive aggressive children had higher cortisol reactivity than proactive and non-aggressive children. Our data suggest that while an overactive HPA-axis response to stress is associated with reactive aggression, stress induced HPA-axis variability does not seem to be related to proactive aggression.