BACKGROUND: Incomplete concordance for psychosis in monozygotic (MZ) twins has been interpreted as indicative of non-genetic cofactors in transmission of the illness. In this case study, we consider childbirth a landmark in the onset of psychotic symptoms, leading to the diagnosis of puerperal psychosis and then to bipolar/schizoaffective disorder. At the end of the third trimester, there is a sudden drop in estrogen, which exerts prominent effects on the serotonergic system in the orbitofrontal cortex (OFC). OBJECTIVES: The purpose of the present study was to investigate OFC activation during emotional processing in MZ twins discordant for affective psychosis. METHODS: Blood-oxygen-level-dependent activation using functional magnetic resonance imaging was measured during the passive viewing of emotional film excerpts. RESULTS: Consistent with our hypothesis, a significant locus of activation was found in the left OFC in the normal MZ twin, but not in the psychosis MZ twin. CONCLUSIONS: The personality changes noted in the psychosis MZ twin (postpartum psychosis) may be related to dysfunctional OFC. Ms J's childbirth may have triggered the onset of psychotic symptoms, leading to the diagnosis of bipolar or schizoaffective disorder.