Cardiac infections after operations are infrequent but, when present, are often fatal. The 14 autopsied patients in whom purulent pericarditis developed after thoracic operations over an 88 year period at The Johns Hopkins Hospital were studied. Purulent pericarditis developed after cardiac operations in 10 and after pulmonary resections in 4. In 12 of the 14 cases the pericardial sac had been opened. Associated postoperative infection, present in 13 patients, included mediastinitis in 7 and empyema in 3. Staphylococcus was the infection organism in half of the patients. Associated cardiac infection, including endocarditis, myocardial abscess, and graft infection, was present in 5 (36 per cent) patients. Death occurred within 2 months of operation in 11 (79 per cent) patients; it was due to infection in 9, cardiac tamponade in 4, and arrhythmias and heart failure in one. The diagnosis of purulent pericarditis was made before death in only 5 (36 per cent) cases, in part owing to masking of the usual signs of pericarditis in the postoperative patient. Since the introduction of antibiotics, the over-all incidence of purulent pericarditis has decreased. However, pericardial infection after thoracotomy has increased tenfold, and patients undergoing cardiac operations in particular provide a new and increasing population at risk for this disease.