BACKGROUND Mediastinitis is a rare, but serious complication of cardiac surgery. It has a significant socioeconomic impact and high morbidity. The purpose of this study was to determine pre-, intra-, and postoperative predictors of mediastinitis. RESULTS In 1700 consecutive patients, who underwent cardiac surgery in 2001, 49 variables were retrospectively assessed. Forty-five patients (2.65%, 95% CI [1.88; 3.41]) developed postoperative mediastinitis. None of these patients died during their hospitalization. Multivariate analysis identified three of the 49 variables as highly significant independent predictors for the development of mediastinitis: obesity (OR 1.03, 95% CI [1.01; 1.04] p = 0.001), chronic obstructive pulmonary disease (OR 3.30, 95% CI [1.58; 6.88], p = 0.001), and bilateral grafting of the internal mammary artery (OR 3.18, 95% CI [1.20; 8.43] p = 0.02). The model is reliable in terms of its goodness of fit, it also discriminates well. Additionally, univariate analysis identified diabetes mellitus, CCS class and the number of intraoperatively transfused units of fresh frozen plasma as variables with a significant impact. CONCLUSIONS The present study suggests that bilateral IMA grafting, chronic obstructive pulmonary disease and obesity are important predictors of mediastinitis.