A 63-year-old Japanese woman was diagnosed with metastatic well-differentiated neuroendocrine carcinoma presenting as a perianal mass without an obvious primary site. Two years later, she presented with a breast mass determined on histologic examination to be the primary neuroendocrine carcinoma. The tumor was weakly positive for estrogen receptor and clearly originated in multifocal ductal carcinoma in situ. At the same time, she was found to have multiple metastases in bone and liver and, later, heart. Most studies report a relatively poor prognosis and limited treatment responsiveness for neuroendocrine breast carcinoma. Better understanding of the cellular origin and molecular pathogenesis of this relatively enigmatic rare disease is required.