A 52-year-old woman with small cell neuroendocrine (oat cell) carcinoma of the breast is described. Identical neoplasms have been reported in a variety of extrapulmonary sites, but this is the first description of a primary mammary tumor of this type. The patient had axillary and hepatic metastases at the time of diagnosis, and the clinical course suggests that this tumor behaves in an aggressive fashion, analogous to other small cell neuroendocrine carcinomas. The relationship of small cell neuroendocrine carcinoma of the breast to mammary "carcinoid tumor" or argyrophilic carcinoma is discussed.