The purpose of the present investigation was to examine the association between circadian rhythms of cortisol and physical and relational aggression. Morning arrival, prelunch, and afternoon predeparture salivary cortisol were assessed among 418 maltreated and nonmaltreated children (52% maltreated; 49% female) attending a summer day camp. Counselors and peers rated participants' involvement in physically and relationally aggressive behaviors. Results indicated that physical aggression was associated with heightened cortisol following morning arrival and relatively steep declines in cortisol over the day, whereas relational aggression was associated with low cortisol following morning arrival and blunted diurnal change in cortisol. Moreover, maltreatment was a significant moderator of this relationship such that aggression was related to greater cortisol dysregulation among nonmaltreated than among maltreated children. The findings suggest that physiological correlates of aggression may differ for physical and relational forms of aggression and among maltreated versus nonmaltreated populations. (Contains 4 tables, 6 figures, and 3 footnotes.)