Primary cardiac osteosarcoma is a rare and aggressive neoplasm that can be difficult to diagnose. We report a case of a previously healthy 49-year-old woman who presented with dyspnea, atrial flutter, and heart failure. A mass was visualized in her left atrium by echocardiography and cardiac computed tomography, and the diagnosis of cardiac myxoma was raised. The patient subsequently underwent surgical resection of the mass and atrial reconstruction. Surprisingly, histological and immunohistological analyses revealed the mass to be an osteosarcoma. The patient received chemotherapy and radiotherapy. Eight months later, she has shown evidence of local recurrence. We briefly discuss primary osteosarcomas in the cardiac cavity and their management< Learning objective: Primary cardiac tumors are very rare and most likely benign. Malignant tumors constitute less than 25% of primary cardiac neoplasms. However, both primary sarcomas and benign tumors are often found in the left atrium. As a consequence of their location and similar clinical presentation, primary cardiac sarcomas can be easily confused with a benign myxoma, therefore abnormal imaging features (immobility of the mass, neovascularity, multicentricity, calcification and invasion into the heart structures) should raise suspicion for a cardiac sarcoma>. © 2012 Japanese College of Cardiology.