A rare case of neuroendocrine small cell carcinoma (SmCC) of the breast is reported. A 51-year-old postmenopausal woman noticed a nodule approximately 3 cm in diameter in her right upper breast. Histologically, the tumor consisted of small ovoid to pleomorphic cells with hyperchromatic nuclei, and a large central area was occupied by acellular amorphous tissue. Extensive lymphatic permeation was seen around the tumor. Invasive and in situ ductal carcinoma foci were not observed in and surrounding the tumor. Immunohistochemically, estrogen and progesterone receptors and HER2/neu were all negative in the tumor cells. Synaptophysin and chromogranin A were diffusely positive in the tumor cells. Cytokeratin 8 was only positive in a few tumor cells. The labeling indices of Ki-67 and p53 were high in the tumor. Postoperatively, systemic studies including positron emission tomography were performed but failed to reveal any other possible primary sites, including lung. Based on these findings, the tumor was diagnosed as neuroendocrine primary SmCC of the breast. Postoperatively, the patient received a course of weekly paclitaxel. However, pelvic bone metastasis was identified on a bone scintigram 1 year after surgery. Mammary SmCC showing high Ki-67 and p53 index should be treated carefully because of their aggressive clinical behavior.