A group of human breast carcinomas shows morphologic and histochemical evidence of neuroendocrine (NE) differentiation. This study presents a structural, immunologic, and electron microscopic analysis of 51 cases in order to establish positive criteria for identification of these tumors, their incidence, variants, and biological behavior. Argyrophilia (by the Grimelius procedure), presence of chromogranin A and/or B, and of synaptophysin are the most reliable histochemical features, correlating with the ultrastructural demonstration of dense-core secretory granules and of clear vesicles of the synaptic type. Structural features alone may be suggestive, but do not prove NE differentiation, which has to be established by additional techniques. Seven histologic types were identified, but those herein described as types A, B, and C, which show cohesive, mucoid, and mixed patterns, respectively, comprise the vast majority of the tumors. Rare NE carcinomas of the breast show structural similarities to Merkel cell and oat cell carcinomas, and behave as highly aggressive tumors. Type B (mucinous) tumors proved to be relatively indolent, while the most frequent types of endocrine tumors (types A and C) have an intermediate grade of malignancy. The analysis of a consecutive series of 100 cases of breast cancer indicates that about 8% of breast carcinomas display some degree of NE differentiation.