Purpose - To examine the clinical progression and innate immune responses during Pseudomonas aeruginosa (PA) keratitis in cathelicidin-deficient (KO) mice. Methods - PA (ATCC 19660) keratitis was induced in KO mice and wild-type (WT) littermates generated on a 129/SVJ background. Clinical score and histopathology were used to monitor the progression of infection at postinfection (PI) days 1, 3, 7, 14, and 21. Mouse corneas were harvested for viable bacteria quantitation, and myeloperoxidase (MPO) assays were performed to determine the number of infiltrating neutrophils. ELISA was used to quantitate interleukin (IL)-1β, IL-6, macrophage inflammatory peptide (MIP)-2, keratinocyte-derived chemokine (KC), tumor necrosis factor (TNF)-α, and vascular endothelial growth factor (VEGF) levels in the corneas. Results - WT mice were resistant (cornea healed), whereas KO mice showed increased susceptibility (corneas failed to recover by 21 days or perforated) to PA infection. Clinical scores were significantly elevated in the infected corneas of KO mice versus WT mice at 7, 14, and 21 days PI. Absence of cathelicidin resulted in significantly delayed clearance of PA in the cornea and an increased number of infiltrating neutrophils at 1, 3, 7, and 14 days PI. KO mice also exhibited differential expression of protein levels for IL-1β, IL-6, MIP-2, KC, TNF-α, and VEGF up to day 21 PI compared with the WT mice. Conclusions - Cathelicidin-deficient mice showed considerable susceptibility to PA keratitis. The present study demonstrates direct in vivo evidence that endogenous expression of cathelicidin provides defense against corneal PA infection indicating its importance in host innate immunity at the ocular surface.