BACKGROUND: There are many causes of pericardial effusion and if the accumulating fluid results in cardiac tamponade, it may lead to life-threatening haemodynamic collapse. Therefore, rapid diagnosis and treatment of cardiac tamponade is critical. CASE SUMMARY: A 53-year-old woman presented to the emergency department with chest pain and undifferentiated shock. A computer tomography (CT) aortogram showed a pericardial effusion, active contrast extravasation (possibly arising from the right coronary artery), but no aortic dissection. Echocardiography confirmed a large pericardial effusion with tamponade physiology. The consensus between cardiology and cardiothoracic surgery was for urgent coronary angiography to identify the source of bleeding. This showed a network of vessels from the right coronary artery that appeared to supply a mass. Re-review of the CT scan and repeat targeted echocardiography showed the silhouette of a mass adjacent to the right atrium. The patient was taken immediately for cardiac surgery. A cardiac tumour extending through the right atrial wall was identified, resected, and subsequently was diagnosed histologically as a cardiac angiosarcoma. DISCUSSION: Malignancy is responsible for only a small proportion of pericardial effusions and metastatic disease is overwhelmingly more common than primary cardiac neoplasms. This case highlights the use of multi-modality cardiac imaging to guide diagnosis and treatment, and the need to consider the rarer causes of haemopericardium in cases where the more common causes have been excluded.