Background: Preterm birth (PTB) is a major cause of neonatal mortality and morbidity. Intrauterine inflammation (IUI) or chorioamnionitis is considered to promote PTB. There is no effective therapy for treating IUI or PTB. Moreover, clinically relevant mechanisms that initiate and propagate IUI remain a fundamental knowledge gap to develop better therapies. Thus, there is a clear need for defining the mechanism for IUI. Activation of IL-1 receptor-associated kinase 1 (IRAK1) is central in engaging immune signaling pathways, including NFkB and MAP kinases, and promotes many inflammatory disorders. Inflammatory mediators such as IL-6, TNF-α and ligands to Toll-like receptors that are known to contribute to IUI, engage and activate IRAK1. However, the role of IRAK1 in IUI remains elusive. Hypothesis: (1) Activation of IRAK1 in the fetal membrane promotes IUI, and (2) Inhibition of IRAK1 prevents IUI and subsequent preterm birth Methods: Pregnant Rhesus macaque were provided intra amniotic LPS (1 mg) or saline at 80% gestation and delivered 16h later. Samples were collected immediately after birth from rhesus delivering preterm. Human fetal membrane samples were collected immediately after birth from women delivering term and preterm birth. Amnion, whole thickness placenta and Decidua parietalis tissues were carefully dissected, homogenized and analyzed by Western blot analysis. Chorioamnionitis was diagnosed by placental histology. Pregnant mice (E 15.5) were pretreated with IRAK1 inhibitor or DMSO and given intraperitoneal ultrapure LPS (250 ug) 4 hours later. Mice (n=13/group) were observed for preterm birth (birth within 48 hours). Results: Phosphorylation of IRAK1 (pIRAK1) is a hallmark for its activation. In Rhesus macaque we observed pathogenesis similar to human chorioamnionitis (Fig 1 A,B). We found elevated IRAK1 phosphorylation and activation of its downstream mediators (TAK1 and JNK) in amniotic and placental tissues in Rhesus IUI (Fig 1 C,D). Consistently, substantial increase in pIRAK1 was observed in amniotic tissues of human preterm subjects with chorioamnionitis compared to term infants without chorioamnionitis (Fig 2A). Similarly, increase in pIRAK1 were observed in amniotic and placental tissues of preterm subjects with chorioamnionitis, compared to preterm subjects without chorioamnionitis. In contrast, very low abundance of pIRAK1 were detected in decidua parietalis tissues from preterm subjects with chorioamnionitis (Fig. 2B). We found a significant reduction in preterm birth in mice treated with IRAK1 inhibitors compared to controls (p<0.001). Conclusion: Our data in human preterm birth, Rhesus and mouse model of preterm birth suggest a compelling novel association of IRAK1 activation in preterm fetal membrane inflammation and subsequent preterm birth.