Sepsis is a systemic inflammatory response to a blood-borne infection that is associated with an extremely high rate of morbidity and mortality. The present study investigates the role of cyclooxygenase (COX)-2 in host responses to bacterial endotoxemia. After administration of Escherichia coli lipopolysaccharide, 50% of wild-type mice die within 96 h. COX-2 deficient mice displayed a dramatic improvement in survival with reduced leukocyte infiltration into critical organs (kidneys and lungs) and a blunted and delayed induction of the cytokine inducible genes nitric oxide synthase 2 and heme oxygenase-1. Translocation and activation of transcription factors important for signaling events during an inflammatory response, such as nuclear factor (NF)-kappaB, were also markedly reduced. While the absence of COX-2 did not alter the induction of several pro-inflammatory cytokines in tissue macrophages, induction of the anti-inflammatory cytokine IL-10 was exaggerated. Administration of IL-10 to wild-type mice reduced NF-kappaB activation. Taken together, our data suggest that COX-2 deficient mice are resistant to many of the detrimental consequences of endotoxemia. These beneficial effects occur, in part, by a compensatory increase in IL-10 that counterbalances the pro-inflammatory host response to endotoxemia.