Neuroendocrine (NE) breast carcinoma is defined by morphological features similar to those of NE tumors of other organs and NE marker expression in at least 50 % of neoplastic cells. However, a NE morphology may be observed even in breast carcinomas lacking NE markers. Human achaete-scute homolog-1 (hASH-1) is a transcription factor that plays a key role in the regulation of mammalian neural and NE cell development and has been identified in several human NE tumors. The aim of this study was to investigate hASH-1 expression in human breast cancers. hASH-1 expression was evaluated in 482 consecutive non-NE invasive breast carcinomas, in a series of 84 breast cancers with >50 % NE marker expression (high NE differentiation) and 21 carcinomas with NE histology but negative or focally (<50 %) positive for NE markers (low NE differentiation). hASH-1 protein was evaluated by a specific monoclonal antibody using immunohistochemistry and gene expression by real-time polymerase chain reaction. None of the non-NE invasive breast carcinomas expressed hASH-1 at any levels. hASH-1 was expressed in tumor cell nuclei of 63 and 38 % of cases with high and low NE differentiation, respectively. Strong correlation with protein and gene expression levels was observed (p < 0.0001). hASH-1 expression was correlated to a low mitotic count (p = 0.02) and a low Ki67 proliferative index (p = 0.0062). hASH-1 expression occurs in breast cancers with NE differentiation regardless of the extent of the NE cell population, and it is restricted to a subset of tumor cells having a low proliferative potential.