Primary neuroendocrine carcinoma of the breast is a rare tumor that comprises less than 1% of breast carcinomas, with most patients being in the sixth or seventh decade of their life. In this article, the authors present the case report of a 42-year-old woman with a rapidly growing tumor in her right breast. After clinical staging by physical examination, breast imaging, and thoracoabdominal computed tomography the patient underwent breast-conserving surgery. The histologic results showed a unique type of high-grade, predominantly large-cell neuroendocrine carcinoma with focal abrupt squamous differentiation. The authors also discuss the clinical and morphological characteristics of the tumor as well as treatment options.