Primary small-cell neuroendocrine carcinoma of the breast is a rare and aggressive neoplasm without an established treatment protocol because so few cases have been described. We report a case of primary small-cell neuroendocrine carcinoma in a 31-year-old woman. The patient came to our hospital 10 days after consulting another clinic, where a diagnosis of locally advanced breast cancer suitable for neoadjuvant chemotherapy had been made. Core needle biopsy under ultrasonographic guidance revealed invasive carcinoma. The doubling time of the tumor progression was calculated as 12 days based on ultrasonographic measurement. After three cycles of chemotherapeutic regimens consisting of adriamycin plus docetaxel, the disease was judged to be progressive and the patient underwent surgery. Definitive histopathological examination revealed primary small-cell neuroendocrine carcinoma. Local and mediastinal recurrence with multiple liver metastases developed only 5 weeks after surgery. Cisplatin plus irinotecan combination chemotherapy was started; however, the patient died of aggressive recurrent tumor progression 6 months after surgery, in spite of the transient tumor regression achieved by chemotherapy. This case reinforces the importance of an early correct diagnosis and the standardization of a treatment regimen for this very rare tumor.