Brain function was evaluated in four psychiatric patients with a history of repetitive purposeless violent behaviour, using EEG, CT scan, and positron emission tomography (PET). Three patients showed spiking activity in left temporal regions, and two showed CT scan abnormalities characterised by generalised cortical atrophy. The PET scans for the four cases showed evidence of blood flow and metabolic abnormalities in the left temporal lobe. Two patients also had derangement in the frontal cortex. The patients showing the largest defects with the PET scans were those whose CT scans were reported as normal. This paper shows the utility of PET in investigating possible brain derangements that could lead to violent behaviour.