Pericardial effusions are a frequently encountered complication of prevalent metastatic malignancies. The incidence of pericardial involvement varies widely from 1.6 to 20% in autopsy studies of patients found to have a metastatic neoplasm [1-3]. Patients with these malignancies are also at an increased risk of thromboembolic disease warranting prolonged courses of anticoagulation therapy . A pervasive belief exists in medicine that anticoagulation increases the risk of cardiac tamponade in patients with known pericardial effusions, especially when the effusions are malignancy related (MR) [5-7]. This is attributed to a perceived increased risk of bleeding into the pericardium, resulting in tamponade physiology. To our knowledge, there are no published data to support this concern. To investigate an association between therapeutic anticoagulation and the development of cardiac tamponade in patients with MR pericardial effusions, we conducted a retrospective, case-control, medical record review of consecutive patients diagnosed with pericardial effusions at Barnes-Jewish Hospital.