OBJECTIVE The mismatch negativity (MMN) to frequency deviant tones has yielded conflicting results in patients with schizophrenia. This might be because Axis I schizophrenia overlaps with Axis II diagnoses such as paranoid or schizotypal personality disorders. This study was designed to address this issue. METHODS We evaluated the auditory MMN to frequency deviance in 17 patients with paranoid, 15 schizotypal, and 16 antisocial personality disorders. These were compared to 25 healthy subjects. RESULTS N1 to both deviant and standard tones was shorter in the paranoid group when compared to healthy controls. MMN latencies were shorter at Fz, Cz, and Pz in the paranoid group when compared to healthy controls, schizotypal, and antisocial groups. MMN amplitudes were higher at Fz and Cz in the schizotypal and antisocial groups when compared to healthy controls and the paranoid group. CONCLUSIONS Patients with paranoid personality disorder had faster automatic detection of auditory stimuli and of their change, but normal inhibition of irrelevant stimuli. By contrast, patients with schizotypal and antisocial personality disorders had normal discrimination of the auditory stimuli, but might have a deficit in inhibition on irrelevant stimuli. Our results might help differentiate these personality types, and clarify some MMN findings in schizophrenia.