Cardiac metastasis of thyroid carcinoma is extremely rare. We treated a case of anaplastic thyroid carcinoma with prominent cardiac metastasis. The 61-year-old male was admitted because of high fever. Investigations revealed a cardiac mass and anaplastic thyroid carcinoma. Resection of the cardiac mass revealed that it was metastasis from the thyroid carcinoma. After 4 months, he died in spite of intensive therapy. Marked leukocytosis was observed during the clinical course, and a concomitant increase of granulocyte macrophage-colony stimulating factor (GM-CSF) level was demonstrated in the sera. It was suggested that the high GM-CSF level in serum contributed to leukocytosis.