The aim of the present study is to classify and to determine the biologic behavior of mammary carcinomas with neuroendocrine differentiation. We applied neuron specific enolase and chromogranin-A immunostaining to the formalin fixed, paraffin embedded sections of 58 mastectomy materials diagnosed as infiltrating ductal carcinoma (24), infiltrating lobular carcinoma (15), mucinous carcinoma (8), mixed mammary carcinoma (4), cribriform carcinoma (3), atypical medullary carcinoma (2), papillary carcinoma (1) and carcinoma with neuroendocrine differentiation (1). Among all specimens seven were chromogranin-A and 32 were neuron specific enolase positive, while four cases revealed positivity with both. Overall 35 cases were neuron specific enolase and/or chromogranin-A positive, which were diagnosed as infiltrating ductal carcinoma (13), infiltrating lobular carcinoma (7), mucinous (6), mixed (3), cribriform (3), carcinoid-like (1), papillary (1) and atypical medullary (1) carcinomas. Positivity was far more common in neuron specific enolase than chromogranin-A. Neuron specific enolase negativity was significantly correlated with blood vessel invasion and chromogranin-A negativity with lymphatic vessel invasion. Most of these tumors (68%) were estrogen and/or progesterone receptor positive. Despite different histologic patterns, these tumors were usually made up of small neoplastic cells and neuroendocrine differentiation was significant among mucinous carcinomas. As up to 60% positivity was detected with these markers in mammary carcinomas, it may be worth to compare positive and negative marker results for the effect on follow-up and the outcome after chemotherapy for definition of its prognostic importance.