Poststernotomy mediastinitis as a complication is rare but disastrous. We assessed incidence, predisposing factors for, and outcome from, mediastinitis after cardiac surgery.We studied 10,713 consecutive patients who underwent open-heart surgery from 1990 to 1999 in a tertiary care university hospital using data prospectively recorded in the hospital discharge register, operating room log, and the hospital's cardiothoracic surgery unit register. Those cases with possible mediastinitis were identified from the hospital infection register and discharge register. Patients' charts were reviewed and cases of mediastinitis confirmed based on criteria of the Centers for Disease Control and Prevention.The overall rate of mediastinitis was 1.1% (120 cases), and higher in coronary artery bypass surgery than in valvular surgery (1.2 vs 0.8%). No trend in incidence was detectable, although surgical patients became progressively older (mean age, 59 to 65 years, p 30 kg/m2), rates of mediastinitis were 0.5%, 1.0%, and 1.8%. In multivariate analysis adjusted for age, sex, year, operation type, and perfusion time, the only predictor for mediastinitis was BMI.Mediastinitis is not diminishing. Larger populations at risk, for example proportions of overweight patients, reinforce the importance of surveillance and pose a challenge in focusing preventive measures.