To identify the risk factors and attributable mortality associated with superinfections in febrile neutropenic patients with hematologic malignancies, we prospectively evaluated 333 episodes of fever and neutropenia by means of univariate and multivariate analyses. Superinfection was defined as any infection either occurring during antibiotic therapy or developing within 1 week after discontinuation of antibiotic therapy. Of 333 episodes, 46 (13.8%) were defined as superinfection; these episodes occurred in 46 patients. The risk factors for superinfection in the multivariate analysis were longer duration of profound neutropenia (P < .0001), lack of use of quinolones as prophylaxis (P < .0001), presence of a central venous catheter (P = .02), and persistence of fever after 3 days of antibiotic therapy (P = .02). The crude mortality rate among patients with superinfection was 48%, and the attributable mortality rate was 24% (95% confidence interval, 3%-45%). Identifying risk factors for superinfections in neutropenic patients might allow clinical practices to reduce the negative impact of this complication.