Anti-estrogens are the current endocrine therapy of choice in the treatment of estrogen receptor (ER)-positive breast cancers. Histone deacetylase inhibitors (HDACi) also constitute a promising treatment for therapy, and combination of anti-estrogens with HDACi may improve efficacy while reducing side effects. We have examined the effect of the HDACi sodium butyrate and suberoylanilide hydroxamic acid (SAHA), alone and in combination with 17beta-estradiol (E2) and the pure anti-estrogen ICI 182.780 (ICI) in human MCF-7 breast cancer cells. HDACi caused a sustained increase of histone H3 acetylation and caused cell death as shown by flow cytometry analysis. In surviving cells, both inhibitors were even stronger than ICI in depleting cyclin D1 levels, inducing expression of the cyclin kinase inhibitor p21Waf1/Cip1, blocking phosphorylation of the retinoblastoma protein, or inhibiting cell growth. No additive effects of ICI with either butyrate or SAHA were found. In addition, these drugs were able to antagonize the effects of E2 on expression of cell cycle proteins, cell growth, and transcription of ER-dependent genes. The anti-estrogenic effects of HDACi appear to be related to a strong downregulation of the expression of ERalpha that appears to be secondary to both transcriptional and post-transcriptional regulation. ERalpha phosphorylation is involved in estrogen signaling, and HDACi also prevented receptor phosphorylation in Ser-118 both in the absence and presence of ER ligands. These results provide further support for the use of deacetylase inhibitors as chemotherapeutic agents in the treatment of breast cancer tumors.