CASE PRESENTATION: A 75-year-old woman presented to our hospital with nausea and vomiting for 3 hours. In the EC, she was found to be hypothermic, hypotensive, and hypoxic. She was tachycardic and tachypneic with prominent jugular venous distension but no peripheral edema. Initial blood work was significant for pH of 7.16, lactic acid of 15.5mg/dL, elevated troponin, creatinine, and transaminases. She required high dose vasopressors and mechanical ventilation. Bedside US revealed large pericardial effusion with diastolic collapse of the right atrium (RA) and the right ventricle. An emergent pericardial window was performed and 450ccs of hemorrhagic pericardial fluid was drained. Pericardial biopsy, cytology, and cultures were unremarkable. Three months after discharge, she returned with similar presentation. Repeat TTE showed a fixed, 7 cm echogenic mass posterior to the RA that extended into the pericardial space and a large effusion. Cardiac MRI showed an asymmetric mass that had T1 and T2 heterogeneous signal intensity and contained a focal papillary projection into the RA and abut the lateral tricuspid leaflet with focal area hyperdensity suggesting a blood clot. She underwent repeat pericardiotomy via median sternostomy for both evacuation of the hematoma and evaluation of the mass; however due to the location of the lesion and the thrombus, only biopsy was performed. Pathology revealed epithelioid angiosarcoma; CD31, CD34, vimentin, and WT-1 positive. Her course of hospitalization was complicated by arrhythmia but all of which self-resolved within 2 weeks post-op. Staging CT ruled out metastasis or other possible primary malignancy. She refused further interventions and was discharged to follow in oncology clinic.