A 65-year-old man with no past history of violent or criminal behavior abruptly strangled his wife after she scratched his face during a domestic argument. He appeared emotionally blunted and lacked insight into the motivations for his violent actions. The presence of left-sided cerebral dysfunction was initially suggested by subtly decreased dexterity of his dominant right hand. A magnetic resonance image (MRI) revealed a large arachnoid cyst centered in his left Sylvian fissure that effaced ventral frontal, anterior temporal and insular cortical gyri. The left middle cerebral artery was displaced relative to an earlier study, providing indirect evidence that the cyst had enlarged during his adult life. Neuropsychological testing indicated generally intact cognitive abilities except for mild impairment on tests of prospective memory and executive functions. He was found to have intermittently decreased EEG amplitude in the left fronto-temporal region. Positron emission tomography (PET) revealed significantly reduced resting glucose metabolism in the left frontal and temporal lobes. He was also found to have pathological diminution of autonomic responses to affectively charged visual stimuli, a phenomenon previously reported in patients with ventromedial frontal lobe injury and acquired disturbances of social conduct. In light of these findings, we suggest that his violent behavior represents stimulus-bound aggression, triggered by a novel physical threat and unchecked by learned social restraints owing to the presence of ventral prefrontal dysfunction.