Primary small cell carcinoma of the breast is an exceedingly rare tumor. The diagnosis can be made with confidence only if other non-breast primary sites can be definitively excluded or if an in-situ component is present. Pulmonary small cell carcinoma can spread to the breast. In rare cases, metastases to the breast may be the first manifestation of an occult pulmonary small cell carcinoma. An important distinguishing feature (besides clinical history) is TTF-1; it is present in lung tumors and absent in mammary tumors.