A review of 310 consecutive cardiac operations with cardiopulmonary bypass for acquired valvular and congenital heart disease found eight cases of late tamponade, 8 to 30 days after operation, and four cases of early tamponade, within 36 hours after operation. Early tamponade, associated with postoperative bleeding, was easily recognized and treated, but late tamponade was often misdiagnosed as cardiac failure or pulmonary embolism. In the eight late cases, six were associated with anticoagulation and hemorrhage, while two developed a bloody effusion with a pericardiotomy syndrome. Tamponade produced an elevated right atrial pressure, low central or mixed venous oxygen tension, oliguria, hypotension, and tachycardia. A severe metabolic acidosis was a late finding. Treatment in the presence of bleeding was uniformly effective if thoracotomy and drainage were promptly instituted. Pericardiocentesis sufficed only in the patients with a pericardiotomy syndrome. Three deaths occurred in the late group from a delay in diagnosis (37.5% mortality).