A 52-year-old woman, who was suffering from aplastic anemia, presented with the clinical features of severe heartfailure. The transthoracic echocardiogram showed a heterogeneous, huge mass on the base of the posterior mitralvalve. We guessed that the mass would be a benign neoplasm and probably myxoma, and we decided upon surgicalresection. After tumor resection, an unexpected result of the histopathology was a high grade osteosarcoma. Theother studies that were done after we had the diagnosis could not reveal any evidence of metastatic malignancy.(Korean Circulation J 2006;36:764-766)