To determine whether antibiotic treatment increases the risk of colonization by Pseudomonas aeruginosa, we performed a case-control study comparing antibiotic exposure in cancer patients colonized by P. aeruginosa and in noncolonized controls. Of 88 patients, 76 had been exposed to at least one antibiotic, but colonization was not statistically associated with exposure to any specific antibiotic treatment, administered orally or parenterally, alone or in combination.