Objectives: To investigate neurophysiological parameters which possibly distinguish subtypes I and II of patients with a bipolar disorder, and contrast the findings with observations from a group of schizophrenic patients and a group of healthy controls. Methods: Sixty-six volunteers underwent a MRI scan to determine the number and location of white matter lesions (WSL). A electrophysiological registration was made while all volunteers performed a auditory 'oddball' task, and the amplitude of the resulting P300 wave was compared. Results: Earlier reports of higher numbers of WSL in bipolar disorder were not replicated in this study. Subtypes I and II showed a different P300 amplitude and subtype I resembled the results of the schizophrenia group. Conclusion: Bipolar patients in remission have a functional brain disorder that is expressed by a change in physiological response to external stimuli.