Granulocytic sarcoma (GS) also known as myeloid sarcoma, chloroma, myeloblastoma, or extramedullary myeloid tumor is a neoplasm composed of immature myeloid cells. The common sites of involvement include bone, central nervous system, soft tissue, lymph nodes, and skin. The involvement of GS in breast tissue is very rare. The incidence of breast GS is 2/1,000,000 in adults. Those affected range in age from 16 to 72 years, with the mean age of 31 years. Primary, isolated, or non-leukemic GS of the breast is defined when bone marrow biopsy confirms the absence of other hematologic malignancy. We here report a case of granulocytic sarcoma of the left breast in a 33 year-old woman who presented with a breast mass. She was initially diagnosed as having diffuse lymphoma, large cell type on Hematoxylin and eosin stain (H&E) histopathology. The tumor cells were, however, strongly positive for myeloperoxidase (MPO), CD117, CD34, and CD43 but negative for CD45, CD20, CD3, or cytokeratin. Although further clinical information, such as complete blood count or aspiration biopsy of bone marrow tissue, was absent, we finally diagnosed this case as GS by additional immunohistochemical study. What happened to the patient?