A retrospective investigation was undertaken of the mediastinal infections in patients submitted to cardiac surgery. A total of 1,763 patients participated in a period of 11 years from 1978 to 1988. All of the case records were reviewed for the occurrence of infections in the mediastinum and 24 patients (1.4%) were found to have had mediastinitis. Predisposing factors to mediastinal infections are reviewed. In all of the cases, treatment consisted re-thoracotomy with removal of all necrotic and infectious material. This was followed by primary closure over a retrosternal irrigation system which supplied an antibiotic solution for six days. In addition, oral antibiotics were administered on the basis of the findings on culture. The incidence of infection was found to be increased when the left internal mammary artery (LIMA) had been employed and in cases of combined cusp and coronary surgery. In 20 patients (83%), relevant culture findings were found from the mediastinal drain on an average of 2.5 days after the primary operation. These were reencountered in the mediastinum at rethoracotomy which was performed on an average of 15 days after the primary operation. This raises the question of the possibility of earlier intervention. The commonest microorganisms were micrococci and Staphylococcus aureus which together comprized approximately 83%. The antibiotic treatment routinely employed was 1 g meticillin four times daily. Five patients died (21%) and 19 patients were discharged. On follow-up examination, the sternum was stable and the scar healed. When infection in the mediastinum is suspected after cardiac surgery, early diagnosis and active surgical treatment are important. The closed method of irrigation drainage is recommended by the authors.