CASE PRESENTATION: 75 year old Japanese woman with history of diabetes, hypertension presented with sudden onset diaphoresis and near syncopal episode. In the emergency room she was very lethargic and in respiratory distress. She was tachycardic, hypotensive with mean arterial pressure of 58 mmHg. Lab work was remarkable for leukocytosis of 25,000 cells/mm3, neutrophils 79%, lactic acidosis, elevated transaminases. She was emergently intubated and started on broad spectrum antibiotics, intravenous fluids, and vasopressors for suspicion of septic shock. A subsequent electrocardiogram showed sinus tachycardia with rate of 107 per min with low voltage complexes, transthoracic echocardiogram (TTE) revealed a large pericardial effusion with tamponade physiology. An emergent pericardial window was performed; 450 ml of bloody fluid was drained. Cytological evaluation of pericardial fluid was negative for malignancy. Biopsy of pericardial sample was unremarkable. Post-procedure, patient had marked improvement of her hemodynamics with resolution of pericardial effusion by TTE. She was discharged home with good condition. Three months later, the patient experienced sudden onset of diaphoresis, chest discomfort, and shortness of breath, which prompted her to seek immediate attention. Her vitals were stable and labs were unremarkable except for mild anemia. A TTE was done showed minimal effusion but computed tomographic angiography (CTA) of chest revealed large localized pericardial effusion, likely hemorrhagic, causing marked mass effect upon superior vena cava (SVC), and right atrium (RA), concerning for tamponade with active extravasation into pericardial effusion; (Fig.1). Cardiac magnetic resonance imaging (MRI) showed 7.4x8.2x6.2 cm mass along the lateral posterior aspect of right atrial wall with acute blood within the mass, blood clot around (Fig.2). Cardiothoracic surgeons performed sternotomy and exploration, found the mass looked like organized hematoma, friable and thinned out right atrial wall with slowly oozing blood. Pericardial hematoma was evacuated, biopsy was performed, and right atrial wall was reinforced and repaired. Histopathologic examination of mass was consistent with Epithelioid Angiosarcoma with an evidence of lymphovascular invasion with pericardial involvement. She opted not to undergo any treatment given dismal prognosis.