OBJECTIVE. To determine the prevalence, risk factors, and outcomes of nosocomial infection due to antimicrobial resistant bacteria in patients treated in the pediatric intensive care unit (PICU). DESIGN. Nested case-cohort study. Patient data were collected prospectively, and antimicrobial susceptibility data were abstracted retrospectively. SETTING. A large pediatric teaching hospital. PATIENTS. All PICU patients admitted from September 1, 1999, to September 1, 2001, unless they died within 24 hours after PICU admission, were 18 years old or older, or were neonatal intensive care unit patients receiving extracorporeal membrane oxygenation. RESULTS. A total of 135 patients with more than 1 nosocomial bacterial infection were analyzed; 52% were male, 75% were white, the mean Pediatric Risk of Mortality score was 10.5, and the mean age was 3.5 years. Of these patients, 37 (27%) had nosocomial infections due to antibiotic-resistant organisms. In univariate analysis, transplantation (odds ratio [OR], 2.83 [95% confidence interval (CI), 1.05-7.66]) and preexisting lung disease (OR, 2.63 [95% CI, 1.18-5.88]) were associated with nosocomial infections due to antibiotic-resistant organisms. Age, Pediatric Risk of Mortality score at admission, length of hospital stay before infection, and other underlying conditions were not associated with infections due to antibiotic-resistant organisms. Patients infected with antibiotic-resistant organisms had greater mean PICU lengths of stay after infection, compared with patients infected with antibiotic-susceptible organisms (22.9 vs 12.8 days; P = .004), and higher crude mortality rates (OR, 2.40 [95% CI, 1.03-5.61]). CONCLUSIONS. Identifiable risk factors exist for nosocomial infections due to antibiotic-resistant organisms. In univariate analysis, infections due to antibiotic-resistant bacteria are associated with increased length of stay in the PICU after onset of infection and increased mortality.