Many symptoms of antisocial personality disorder have been proposed to be related to decreased daytime vigilance. To explore this hypothesis, quantitative analyses were conducted of the electroencephalographic (EEG) activity of drug-free and detoxified homicidal male offenders with antisocial personality disorder as the primary diagnosis. Subjects comprised 16 men recruited from a forensic psychiatric examination in a special ward of a university psychiatric hospital. Fifteen healthy age- and gender-matched controls with no criminal record or history of physical violence consisted of hospital staff and students. An overall reduction of alpha power was observed in the waking EEG of offenders. A bilateral increase in occipital delta and theta power was also found in these individuals. This study provides further support to the growing evidence of brain dysfunction in severe aggressive behavior. Homicidal offenders with antisocial personality disorder seem to have difficulties in maintaining normal daytime arousal. Decreased vigilance, together with social and psychological variables, may explain their aberrant behavior in everyday life. New studies are, however, needed to specify the vigilance problems of this patient group.