Estrogen is well-established as a mitogenic factor implicated in the tumorigenesis and progression of breast cancer via its binding to the estrogen receptor alpha (ERalpha). Recent data indicate that chromatin inactivation mediated by histone deacetylation (HDAC) and DNA methylation is a critical component of ERalpha silencing in human breast cancer cells. The aim of this study was to determine the expression of the HDAC1 gene in malignant human breast tissue and to correlate our observations with available clinical information. In the present study, the level of expression of HDAC1 mRNA was assessed by LightCycler-based quantitative real-time reverse transcriptase (RT)-PCR analysis in 162 cases of invasive carcinoma of the breast. Associations between HDAC1 mRNA expression and different clinicopathological factors were sought. It was found that HDAC1 mRNA was expressed at significantly higher levels in tumors from patients over 50 years of age and in those tumors without axillary lymph node involvement, that are less than 2 cm, that are of a non-high histological grade, that are HER2 negative and that are ERalpha/PgR positive. Patients with tumors displaying high levels of HDAC1 mRNA expression tended to have a better prognosis in terms of both disease-free and overall survival. However, univariate and multivariate analysis did not show HDAC1 mRNA expression level to be an independent prognostic factor for either disease-free or overall survival. These results imply that HDAC1 mRNA expression could have potential as an endocrine response marker and may have prognostic implications for breast cancer progression.