A case of primary small cell (oat cell) carcinoma of the breast in a 41-year-old woman is presented. The patient was alive and well without disease 16 months after modified radical mastectomy and subsequent chemotherapy. The tumor cells revealed morphologic similarity to oat cell carcinoma of the lung and immunohistochemical expression of neuroendocrine markers. In ultrastructural examination, the tumor cells had neurosecretory granules. Review of nine previously reported cases and this case of primary small cell carcinoma of the breast has revealed that this type of tumor shows prominent vascular invasion, frequent lymph node metastasis, infrequent expression of estrogen receptor, and also very poor prognosis. Immunohistochemical study for the c-kit proto-oncogene product, which has been reported to be a specific marker for pulmonary small cell carcinoma, demonstrated positive reactivity in approximately 80% of the tumor cells of this case, which is the first report according to our knowledge. The expression of c-kit might be some aid to the diagnosis of primary small cell carcinoma of the breast.