A 52-year-old woman presented with a palpable mass in the upper-outer quadrant of her left breast. Ultrasonography revealed a mass measuring 11×12×8 mm in size in the left breast. Core needle biopsy was performed, and immunohistochemical examination revealed positive staining for AE1/AE3, CAM 5.2, CK7, CD56 and synaptophysin. On the basis of the findings, we made the diagnosis of an invasive neuroendocrine carcinoma, possibly small cell carcinoma of the breast. The patient was treated by left mastectomy and axillary lymphadenectomy. Our final pathological diagnosis was primary small cell carcinoma of the breast. Small cell carcinoma of the breast is very rare. We report this case together with a review of the literature.