We describe a case of cardiac myxoma in a 44-year-old Japanese man, who died after developing metastases in the skin, brain and muscle. A satellite tumor which was attached to the wall of the abdominal aorta induced marked hypertension due to obstruction of the renal arteries. Although the primary heart tumor had typical histological features of benign cardiac myxoma, the recurrent heart tumor, which was partly resected three months before the patient's death, showed apparently malignant characteristics resembling malignant fibrous histiocytoma (MFH). Since the histological features of the initial and recurrent tumors were different, the grade of malignancy was investigated using the cellularity of the tumor as an arbitrary criterion. A gradual but significant increase in the cellularity was observed over the course of five years. Immunohistochemically, tumor cells in the muscle metastasis contained vimentin and factor VIII-related antigen, and multinucleated giant cells in the recurrent heart tumor contained desmin, which is rarely detectable in MFH. Therefore, we considered that the present case represented malignant transformation of benign cardiac myxoma.