One of the most common causes of pericardial effusion in the Western world is malignancy. Emergency physicians must maintain vigilance in suspecting pericardial effusion and tamponade in patients with known or suspected malignancy who present with tachycardia, dyspnea, and hypotension. Diagnosis can be expedited by key physical examination, electrocardiogram, and sonographic findings. Unstable or crashing patients with tamponade must undergo emergent pericardiocentesis for removal of fluid and pressure to restore cardiac output.