The aim of the retrospective case-control study presented here was to elucidate the incidence, risk factors, and outcomes of nosocomial infections caused by quinolone-resistant Escherichia coli (QREC). During the 3-year period studied, 51 nosocomial QREC infections were found, and the characteristics of these cases were compared with those of 102 control patients with quinolone-susceptible nosocomial infections. In the multivariate analysis, risk factors were identified as prior quinolone therapy (odds ratio [OR], 18.49; 95% confidence interval [CI], 5.53-61.82; P value <0.001), urinary tract abnormalities (OR, 6.69; 95% CI, 1.68-26.63; P=0.007), and prior therapy with other antimicrobial agents (OR, 3.57; 95% CI, 1.38-9.27; P=0.009). No difference in mortality or in length of hospital stay was found. Prudent use of quinolones, especially in patients with urinary tract abnormalities, is probably the best way to avoid an increase in the incidence of QREC infections, but further studies on interventions with restricted use of quinolones are necessary to demonstrate the effectiveness and safety of this strategy.