Spontaneous rupture of a primary cardiac angiosarcoma is a rare condition with a poor prognosis. The authors describe the case of a 48-year-old man with abnormal blood flow from a pericardial mass to the right atrium 3 months after pericardiocentesis. The flow was presumed to have occurred due to rupture of this cardiac angiosarcoma of the right atrial wall and pericardium into the right atrial cavity.