Neuroendocrine carcinoid tumours of the breast: a variant of carcinoma with neuroendocrine differentiation. Carcinoid tumours most frequently develop in the gastrointestinal tract but have been described in many organs of the body. In 1977 the first cases were reported in the mamma, followed by descriptions of argyrophilic carcinoid-like, neuroendocrine mammary tumours by many investigators who performed immunohistochemical and ultrastructural examinations. The existence of true carcinoids in the mamma is still a controversial issue. Eight mammary neoplasms with monomorphous cytonuclear features, five of the small cell carcinoid-like variety and three composed of larger cells, were examined by immunohistochemical and ultrastructural examination. We believe this kind of tumours are ductal or lobular carcinomas with focal or more extensive neuroendocrine features and are the result of a dual differentiation of neoplastic precursor stem cells along epithelial and endocrine lines. Consequently, we consider that treatment of such cases should not be different from that of the ordinary type of mammary carcinomas. 