The aim of the present study was to investigate P300 event-related potential components in euthymic bipolar disorder type I (BDI) and bipolar disorder type II (BDII) patients and matched controls. A total of 10 BDI patients, 10 BDII patients and 10 healthy individuals were enrolled in the study. Event-related potential data were collected according to a standard auditory 'oddball' paradigm. A significant groups effect in both the peak amplitude (P<0.001) and the mean amplitude (P<0.001) was observed; post-hoc comparisons showed that the peak and mean amplitudes of BDI and BDII patients were significantly lower than the peak and mean amplitudes of the healthy controls. The neurophysiological patterns found in the present study might at least partially reflect the presence of a mild selective cognitive impairment in euthymic BDI and BDII patients. From a clinical point of view, these evidences support the potential role of cognitive interventions in the treatment of BD.