The recognition of undifferentiated forms of endocrine tumors in the extrapulmonary sites is considered as extremely infrequent. Immunohistochemical patterns of this tumor are similar to small cell neuroendocrine cancer of the lung and as the pulmonary one, it is more aggressive than carcinoma without neuroendocrine differentiation. We report a case of a 68 years old woman with a primary small cell carcinoma of the breast gland presented as a palpable and mobile 2 cm mass, located in the upper outer quadrant of her right breast. It was treated with surgery and the diagnosis was made after surgical treatment, thanks to the immunohistochemical studies of tissue.