Granulocytic sarcoma is an extramedullary tumor composed of immature granulocytic cells ; metastasis to the breast is extremely rare. We describe a case of metastatic granulocytic sarcoma of the breast. The primary site of the lesion was the sixth thoracic vertebra ; there was no evidence of leukemia in peripheral blood or bone marrow. Mammography showed a 2cm-sized high-density mass lesion in the upper portion of the left breast, while breast ultrasonography revealed an oval-shaped mixed heterogeneous hypoechoic mass with mild posterior enhancement.