Current evidence indicates that dysregulation of the host inflammatory response to infectious agents is central to the mortality of patients with sepsis and in those with systemic inflammatory response syndrome. Strategies to block inflammatory mediators, often with complicated outcomes, are currently being investigated as new adjuvant therapies for sepsis. Here, we determined if administration of recombinant platelet-activating factor (rPAF)-acetylhydrolase (rPAF-AH), an enzyme that inactivates PAF and PAF-like lipids, protects mice from inflammatory injury and death after administration of lipopolysaccharide (LPS) or cecal ligation and puncture (CLP). Administration of rPAF-AH increased plasma PAF-AH activity and reduced mortality in both models. Treatment with rPAF-AH increased peritoneal fluid levels of monocyte chemoattractant protein 1/CCL-2 and decreased interleukin 6 and migration inhibitory factor levels after LPS administration or CLP. Administration of a broad-spectrum antibiotic together with rPAF-AH was more protective than single treatment with either of these agents. The combined treatment was associated with reduced interleukin 6 levels in mice subjected to CLP. We observed acute decreases in plasma PAF-AH activity in mice subjected to CLP or challenged with LPS and in human patients with sepsis. We conclude that alterations in the endogenous PAF-AH contribute to the pathophysiology of sepsis and that administration of exogenous rPAF-AH reduces inflammatory injury and mortality in models relevant to the clinical syndrome. Variations in endogenous PAF-AH activity may potentially account for variable responses to exogenous rPAF-AH in previous clinical trials. Serial measurements of plasma PAF-AH activity in murine models demonstrate dynamic regulation of the endogenous enzyme, potentially explaining the variations in human subjects.