BACKGROUND Postoperative mediastinitis is one of the most feared complications in patients who undergo cardiac surgery because in addition to a high mortality rate (10% to 47%), there are increases in the length of hospital stay and in hospital costs. The purpose of the present study is to assess the risk factors for mediastinitis after cardiac surgery, the mediastinitis rate, and the mortality rate in our institution. METHODS To determine the risk factors, a matched case-control study was carried out, with 39 cases and 78 controls, among the patients who underwent cardiac surgery at the Dante Pazzanese Cardiology Institute, São Paulo, Brazil. RESULTS In the period of the study, 9,136 cardiac surgeries were performed and the mediastinitis rate was 0.5%. In the multivariate analysis, the independent risk factors found were obesity (odds ratio, 6.49; 95% confidence interval, 2.24 to 18.78), smoking (odds ratio, 3.27; 95% confidence interval, 1.04 to 10.20), intensive care unit stay more than 2 days (odds ratio, 4.50; 95% confidence interval, 1.57 to 12.90), and infection at another site (odds ratio, 8.86; 95% confidence interval, 1.86 to 42.27). The mortality rate was 23% among the patients with mediastinitis. CONCLUSIONS We observed two independent risk factors related to patients' antecedents (obesity and smoking) and two risk factors related to problems in the postoperative period (length of intensive care unit stay and infection at another site). Efforts should be concentrated so that patients lose weight and stop smoking before elective surgeries. There should also be a prevention program against hospital infection directed to, and intensified for, at-risk patients.