The authors assessed aggressive behavior in 89 patients with traumatic brain injury (TBI) and 26 patients with multiple trauma but without TBI using a quantitative scale (the Overt Aggression Scale) and examined its clinical correlates. Aggressive behavior was found in 33.7% of TBI patients and 11.5% of patients without TBI during the first 6 months after injury. Aggressive behavior was significantly associated with the presence of major depression, frontal lobe lesions, poor premorbid social functioning, and a history of alcohol and substance abuse. Interventions aimed at treatment of depression and substance abuse and enhancing social support may help reduce the severity of this disruptive behavior.