Acute pericardial tamponade is a potentially life-threatening condition that requires immediate treatment. This report describes a patient who presented to the emergency department with an acute hemopericardium and echocardiographic evidence of cardiac tamponade following the initiation of warfarin therapy for a recently diagnosed pulmonary embolism. The association between cardiac tamponade, oral anticoagulation and pulmonary thromboembolic disease is briefly discussed.