Malignant primary cardiac tumors are rare with the most common type among them being sarcomas. However, a myxoid sarcoma in the heart is very rare and differentiating it from from cardiac myxoma is often difficult. Here, we report a case of rapid regrowth of a left atrial tumor after surgical resection that was finally diagnosed as cardiac myxoid fibrosarcoma. An 82-year-old man, who underwent resection of a cardiac tumor 3 months ago, presented with severe dyspnea and peripheral edema. He was diagnosed with a mitral valve obstruction due to the regrowth of a huge left atrial tumor. The patient had a second resection and the resected tumor was finally diagnosed as myxoid fibrosarcoma.