Using a classical click/flash paradigm the contingent negative variation (CNV) was recorded from 60 consecutive male admissions to Broadmoor Hospital. Clinical and psychometric Minnesota Multiphasic Personality Inventory (MMPI) assessments were made independently of the electroencephalography (EEG) data. A trend for higher amplitude CNV's to be recorded from patients with a clinical diagnosis of antisocial (psychopathic) personality was observed. Patients with a Mental Health Act diagnosis of psychopathic disorder had significantly higher voltage CNV's than those in the mental illness category. These CNV differences were only found in the primary (non-anxious, sociable) psychopaths. Secondary (neurotic, unsociable) psychopaths did not differ from staff controls. A positive association between CNV voltage and the personality factor of sociability(sociable-shy dimension) was observed. A weak, negative correlation between CNV amplitude and impulsivity (impulsive-controlled dimension) was also apparent. All subjects showed an habituation effect across trials. No difference in response variability between successive series of trials was apparent in any of the patient groups.