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Background: Accumulating evidence suggests that dysregulation of the immune system represents an important vulnerability factor for mood disorders. Postpartum psychosis (PP) is a severe mood disorder occurring within 4 weeks after delivery, a period of heightened immune responsiveness and an altered endocrine set point. Therefore, the aim of this study was to examine immune activation in patients with first-onset PP at the level of monocytes, T cells, and serum cytokines/chemokines. Methods: We included 63 women admitted with first-onset PP. Control groups included healthy postpartum (n = 56) and nonpostpartum (n = 136) women. A quantitative-polymerase chain reaction monocyte gene expression analysis was performed with 43 genes previously identified as abnormally regulated in nonpostpartum mood disorder patients including the isoforms of the glucocorticoid receptor. Peripheral blood mononuclear cells percentages were measured by fluorescence-activated cell sorter analysis, whereas serum cytokines/chemokines were determined with a cytometric bead array. Results: In healthy women, postpartum T cell levels were significantly elevated compared with nonpostpartum. Patients with PP failed to show the normal postpartum T cell elevation. In contrast, these patients showed a significant elevation of monocyte levels and a significant upregulation of several immune-related monocyte genes compared with control subjects postpartum and nonpostpartum. Furthermore, the glucocorticoid receptor α/β gene expression ratio was decreased in monocytes of PP patients, strongly correlating with their immune activation. Conclusions: This study demonstrates a robust dysregulation of the immuno-neuro-endocrine set point in PP, with a notable over-activation of the monocyte/macrophage arm of the immune system. [Copyright &y& Elsevier]