To determine the importance of Pseudomonas aeruginosa exotoxin A (ETA) as a virulence factor in corneal disease.Isogenic mutants deficient in ETA were constructed in P. aeruginosa strains PAO1 and ATCC 19660 by allelic exchange and then evaluated for virulence in a mouse model of bacterial keratitis. The effect of ETA on adherence to scarified corneal epithelium was assessed in an in vitro organ culture model.Mutants of either P. aeruginosa PAO1 or 19660, deficient in ETA, adhered to wounded corneal tissue and initiated ocular disease similar to that in wild-type strains. However, in contrast to wild-type strains, ETA mutants were quickly cleared from the eye, inflammation diminished, and the cornea healed.Although ETA has no effect on the ability of P. aeruginosa to adhere to corneal wounds or to initiate Pseudomonas keratitis, it is crucial for the organism to persist in the eye and ultimately cause disease.