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Mediastinitis, although an infrequent complication of median sternotomy, represents a significant source of morbidity and mortality.To determine the incidence and describe the epidemiology and microbiology of mediastinitis in children after cardiac surgery and to identify risk factors for the development of Gram-negative mediastinitis.This was a retrospective case-control study nested within the cohort of children, birth to 18 years of age, undergoing median sternotomy between January 1, 1995 and December 31, 2003.Forty-three cases of mediastinitis were identified. The incidence of mediastinitis was 1.4%. Median patient age at time of inciting sternotomy was 32 days (interquartile range, 5 days–9 months). Twenty-three (54%) cases occurred in girls. Median time to onset of infection after surgery was 11 days (range, 4–34 days). Overall Gram-positive organisms were present in 29 (67%) cases, and Gram-negative organisms were present in 13 (30%) cases. The organisms most commonly isolated from mediastinal culture were Staphylococcus aureus (46%), coagulase-negative staphylococci (17%) and Pseudomonas aeruginosa (17%). The rate of concurrent bacteremia was 53% (95% confidence interval, 38–69%). In multivariable analysis, delayed sternal closure was an independent risk factor for the development of Gram-negative mediastinitis (odds ratio, 9.3; 95% confidence interval, 1.5–56.8; P = 0.016).Although Gram-positive organisms were the most common cause of infection, Gram-negative organisms accounted for one-third of all isolates. More than one-half of patients with mediastinitis had concurrent bacteremia. Delayed sternal closure was an independent risk factor for Gram-negative mediastinitis.