The incidence of acute suppurative mediastinitis after open heart cardiac surgery in a 5 year retrospective study was found to be 1.56%. The commonest causal agent was the DNAse + coagulase + staphylococcus (68.8%). All cases were treated with bactericidal antibiotics and local measures (dosed chest irrigation-drainage in 1 or 2 stages). Adult men, early and/or late reoperation, immediate postoperative complications (cardiovascular collapse, haemorrhage) were positive risk factors. On the other hand, the type of underlying cardiac disease, the duration of surgery and bypass time, emergency surgery, and protocols of prophylactic antibiotic therapy did not seem to influence this risk. No sources of exogenous contamination, no notions of epidemics or periodicity were discovered in this study. The clinical course may be complicated, especially by renal failure. Thirty-two of the 109 patients studied died. However, the prognosis has improved in the last 5 years in relation to technical advances in intensive care and, above all, to adoption of 1 stage dosed chest irrigation-drainage: the mortality rate has fallen from 37.2% in the first period to 20% more recently. When a cure is obtained, it is complete and there are no sequellae. Nevertheless, irrespective of the adoption of draconian measures of asepsis and protocols of prophylactic antibiotic therapy, the risk of acute postoperative mediastinitis persists. This suggests that the efficacy of the patient's local (and general) anti-infective immunity defenses may vary in the presence of constant mediastinal bacterial contamination despite aleatory preventive measures, due to the remenance of the skin flora.