In a matched case-control study, we studied the effect of prior receipt of fluoroquinolones on isolation of three third-generation cephalosporin-resistant gram-negative nosocomial pathogens. Two hundred eighty-two cases with a third-generation cephalosporin-resistant pathogen (203 with Enterobacter spp., 50 with Pseudomonas aeruginosa, and 29 with Klebsiella pneumoniae) were matched on length of stay to controls in a 1:2 ratio. Case-patients and controls were similar in age (mean 62 years) and sex (54% male). Variables predicting third-generation cephalosporin resistance were surgery (p = 0.005); intensive care unit stay (p < 0.001); and receipt of a b-lactam/b-lactamase inhibitor (p < 0.001), a ureidopenicillin (p = 0.002), or a third-generation cephalosporin (p < 0.001). Receipt of a fluoroquinolone was protective against isolation of a third-generation cephalosporin-resistant pathogen (p = 0.005). Interventional studies are required to determine whether replacing third-generation cephalosporins with fluoroquinolones will be effective in reducing cephalosporin resistance and the effect of such interventions on fluoroquinolone resistance.