Patients with cystic fibrosis (CF) develop chronic Pseudomonas aeruginosa lung infection with mucoid strains of P. aeruginosa; these infections cause significant morbidity. The immunologic response in these infections is characterized by an influx of neutrophils to the lung and subsequent lung damage over time; however the underlying mediators to this response are not well understood. We recently reported that IL-23 and IL-17 were elevated in the sputum of patients with CF who were actively infected with P. aeruginosa; however, the importance of IL-23 and IL-17 in mediating this inflammation was unclear. To understand the role that IL-23 plays in initiating airway inflammation in response to P. aeruginosa, IL-23p19-/- (IL-23 deficient) and wild-type (WT) mice were challenged with agarose beads containing a clinical, mucoid isolate of P. aeruginosa. Levels of proinflammatory cytokines, chemokines, bacterial dissemination and inflammatory infilitrates were measured. IL-23 deficient mice had significantly lower induction of IL-17, KC, and IL-6, decreased BAL neutrophils, MMP-9, and reduced airway inflammation than WT mice. Despite the reduced level of inflammation in IL-23p19-/- mice there were no differences in the induction of TNF and interferon-gamma or in bacterial dissemination between the two groups. This study demonstrates that IL-23 plays a critical role in generating airway inflammation observed in mucoid P. aeruginosa infection and suggests that IL-23 could be a potential target for immunotherapy to treat airway inflammation in CF.