Background. Nitric oxide (NO) produced by the inducible isoform of NO synthase (iNOS or NOS2) has been implicated in the hypotension, organ failure, and death that complicate sepsis. To avoid the confounding effects and limitations of iNOS inhibitors, we used iNOS gene 'knockout' mice to examine the effect of inducible NO production in a model of polymicrobial abdominal sepsis treated with antibiotics. We hypothesized that iNOS gene deficiency would significantly alter outcome. Methods. C57BL6 wild-type (control) and congenic iNOS knockout mice were studied concurrently. Under halothane anesthesia, the ceca were ligated with 4-0 silk suture and punctured twice with a 26-gauge needle (cecal ligation and puncture, CLP). Survival was followed for 7 days, after which necropsies were performed in surviving animals. In an accompanying study examining the acute effects of sepsis, organ injury at 18 hours after CLP as determined by histology and the degree of cell death by apoptosis were examined with the use of hematoxylin and eosin (H and E) and TUNEL staining and two-channel fluorescence-activated cell sorter (FA CS) analysis. Results. Sham laparotomy produced no lethality in either knockout (n = 3) or wild-type (n = 3) animals. Compared with survival in controls (n = 20), survival after CLP in iNOS knockout mice (n = 21) was significantly decreased (P < .01 at 2 days, P = .080 at 7 days, Mantel-Haenszel log-rank test). CLP-induced apoptotic cell death was significantly less in the thymus of iNOS knockout mice compared with wild- type mice. Conclusions. We conclude that iNOS gene function provides a survival benefit in septic mice and is associated with increased sepsis- induced thymocyte apoptosis. To our knowledge, this is the first survival study examining the effect of iNOS gene deficiency in a clinically relevant model of sepsis.