Transoesophageal echocardiography disclosed a localized pericardial blood clot compressing the right atrium (RA) and/or right ventricle (RV) in 15 patients suffering from low cardiac output failure soon after open-heart surgery. The left ventricular end-diastolic diameter was small (38.4 +/- 10.1 mm) and its fractional shortening normal (34.9 +/- 10.2%). These findings suggested cardiac tamponade as a result of pericardial clot. However, the 'y' trough of the RA pressure tracing was prominent, which is not characteristic of typical cardiac tamponade, but rather of constrictive pericarditis. This implies therefore that the pathophysiology of cardiac tamponade by pericardial clot differs from that of tamponade by fluid. Emergency open-chest removal of the pericardial clot was performed in seven patients, with good results. Pericardial clot produces low cardiac output soon after open-heart surgery, but its location is specific and its haemodynamics are not characteristic of cardiac tamponade.