Members of the signal transducer and activator of transcription (STAT) family are transcription factors that mediate many of the effects of pro- and anti-inflammatory cytokines. The progressive systemic inflammatory response induced by endotoxin is mediated by overzealous cytokine production. Here we identify STAT4 and STAT6 as critical regulators of the systemic inflammatory response to endotoxin. Mice deficient for STAT4 or STAT6 were highly susceptible to lethal endotoxemia. In STAT4(-/-) mice, antibody blockade of IL-12 prevented mortality, suggesting that STAT4 confers protection, while another signaling pathway mediates the detrimental effects of IL-12. In STAT6(-/-) mice we observed dysregulated activation of the transcription factor NF-kappaB, resulting in augmented production of proinflammatory cytokines and chemokines. Furthermore, STAT6(-/-) mice displayed increased organ accumulation of leukocytes and significant hepatocellular injury. These findings demonstrate that STAT4 and STAT6 confer protection against endotoxin-induced death and that for STAT6 these protective effects occur through the regulation of NF-kappaB activation and subsequent production of proinflammatory cytokines and chemokines.