Primary small cell carcinoma of the breast is exceedingly rare, with fewer than 25 reported cases. The case presented herein is that of a 61-year-old woman with a 2.5-cm mass of the left breast. She underwent mastectomy with axillary node dissection. Histologic examination revealed sheets and nests of small, hyperchromatic, malignant cells with indistinct nucleoli and scant cytoplasm. High-grade solid and comedo ductal carcinoma in situ also was present. Two of 5 axillary lymph nodes contained metastatic disease. Immunohistochemical analysis demonstrated weak immunoreactivity for cytokeratin, neuron-specific enolase, and bcl-2. This histologic and immunohistochemical profile was consistent with that of a primary small cell carcinoma. Interestingly, this neoplasm lacked immunoreactivity for E-cadherin. E-cadherin expression has been documented in all 11 (100%) of 11 previously reported cases of primary small cell carcinoma of the breast, suggesting that this tumor is a form of ductal carcinoma. To our knowledge, this is the first reported case of E-cadherin-negative small cell carcinoma of the breast, which raises the question of a possible lobular histogenesis in some of these neoplasms.