A series of neuropsychological tests and an EEG examination were given to three groups of male adolescents who were inpatients in a psychiatric hospital. The groups were as follows: group 1: a group of seven patients with a diagnosis of behavior disorder, each with a history of multiple assaultive incidents; group 2: a group of eight patients with a diagnosis of behavior disorder, each with a history of zero to two assaultive incidents; group 3: a group of six patients with a diagnosis of organic brain syndrome (OBS). It was hypothesized that the repetitively assaultive behavior disorder patients would be more similar to the OBS patients than to the nonassaultive behavior disorder patients. The data revealed that several variables distinguished the nonassaultive behavior disorder patients from both OBS patients and assaultive behavior disorder patients; the latter two groups were indistinguishable from each other. The significant variables were: a) EEG abnormality; b) degree of establishment of hand dominance; c) perseveration errors in a visuomotor task; d) verbal short term memory. Paroxysmal activity, particularly in the frontal area, appeared to be the most important EEG feature related to assaultive behavior. The neuropsychological data give some support to the hypothesis that severe aggression is related to left hemisphere dysfunction. It is concluded that neuropsychological assessment can reveal organic features important in understanding repetitively violent behavior.