The Pseudomonas aeruginosa quorum-sensing systems,las and rhl, control the production of numerous virulence factors. In this study, we have used the burned-mouse model to examine the contribution of quorum-sensing systems to the pathogenesis of P. aeruginosa infections in burn wounds. Different quorum-sensing mutants of P. aeruginosa PAO1 that were defective in the lasR, lasI, orrhlI gene or both the lasI and rhlIgenes were utilized. The following parameters of the P. aeruginosa infection were examined: (i) lethality to the burned mouse, (ii) dissemination of the P. aeruginosa strain within the body of the infected mouse (by determining the numbers of CFU of P. aeruginosa within the liver and spleen), and (iii) spread of the P. aeruginosa strain within the burned skin (by determining the numbers of CFU of P. aeruginosa at the inoculation site and at a site about 15 mm from the inoculation site [distant site]). In comparison with that of PAO1, the in vivo virulence of lasI, lasR, and rhlImutants was significantly reduced. However, the most significant reduction in in vivo virulence was seen with the lasI rhlImutant. The numbers of CFU that were recovered from the livers, spleens, and skin of mice infected with different mutants were significantly lower than those of PAO1. At 8 and 16 h post burn infection, comparable numbers of CFU of PAO1 and lasI andrhlI mutants were obtained from both the inoculation and distant sites of the burned skin of infected mice. In contrast, CFU of the lasR mutant and the lasI rhlI double mutant were recovered only from the inoculation site of infected mice at 8 and 16 h post burn infection. The ability of a plasmid carrying either the lasI or rhlI gene or the lasIand rhlI genes to complement the defect of the lasI rhlI double mutant was also examined. The presence of any of these plasmids within the lasI rhlI double mutant significantly enhanced its in vivo virulence, as well as its ability to spread within the burned skin. These results suggest that the quorum-sensing systems play an important role in the horizontal spread of P. aeruginosa within burned skin and in the dissemination of P. aeruginosa within the bodies of burned-and-infected mice and contributed to the overall virulence ofP. aeruginosa in this animal model.