To determine the incidence of, and risk factors for, nosocomial infections in neonates during and after treatment with extracorporeal membrane oxygenation (ECMO).A retrospective cohort study of all infants treated with ECMO during a 4-year period.26 neonates experiencing 32 nosocomial infections, and 54 neonates with no nosocomial infections.A large, urban children's hospital.30% of neonates treated with ECMO experienced at least one nosocomial infection. The rate of nosocomial infections was 10.3 per 1,000 patient-days. Risk factors significantly associated with nosocomial infection included duration of ECMO > 7 days (mean, 12 vs 7 days; odds ratio [OR], 2.84; 95% confidence interval [CI95], 0.96-8.56); neonatal intensive care-unit stay > 21 days (mean, 44 vs 19 days; OR, 8.73; CI95, 2.49-31.94); hospitalization > 50 days (mean, 63 vs 29 days; OR, 5.44; CI95, 1.47-20.87); and surgical procedure before or during ECMO (46% vs 22%; OR, 4.58 CI95, 1.25 to 17.38).Nosocomial infections occurred in 30% of patients undergoing ECMO. Although nosocomial infections were not associated with an increase in mortality, hospitalization was prolonged.