This is a case of granulocytic sarcoma presenting as bilateral breast masses in a 40-yr-old woman with concurrent unsuspected chronic myeloid leukemia diagnosed by fine-needle aspiration. The granulocytic differentiation was recognized on Diff-Quik-stained cytology smears and confirmed rapidly on flow cytometry on the same day. The breast has been reported to be an uncommon site for granulocytic sarcoma. We found that 38.8% of granulocytic sarcomas diagnosed by fine-needle aspiration in the English-language literature occurred in the breast. In the absence of clinical history or hematological abnormality, granulocytic sarcoma may be misdiagnosed, depending on the degree of myeloid differentiation present within the tumor. The differential diagnosis includes large-cell non-Hodgkin's lymphoma, lobular carcinoma of the breast, undifferentiated carcinoma, malignant melanoma, extramedullary hemopoiesis and inflammation. The key morphological features and useful ancillary tests are discussed.