OBJECTIVES: Diminished suppression of the P50 response, a consistent finding in schizophrenia, has also been reported in patients with bipolar disorder. It is a promising endophenotype for schizophrenia, but its relationship to genetic liability in mood disorder is controversial. The present study investigated event-related brain potential (ERP) indices of auditory processing and sensory gating in mood disorder and subgroups of patients with bipolar disorder with or without a history of psychosis using the P50 dual-click procedure. METHODS: The P50 auditory evoked potential response to paired stimuli was measured in 77 subjects with mood disorder (58 with bipolar disorder and 19 with major depressive disorder) and 28 healthy controls. P50 parameters were compared between groups. RESULTS: P50 suppression in patients with mood disorder did not differ from that in the healthy subjects. Except for S1 latency, there were no significant differences in P50 parameters between the diagnosis groups. In patients with bipolar disorder, a history of psychosis made no difference to P50 parameters. CONCLUSION: P50 was not significantly impaired in patients with mood disorder. There has been much debate on the meaning of P50, and more studies including longitudinal within-subjects studies are warranted to clarify the meaning and mechanisms of P50.