Estrogen suppression is an effective endocrine treatment option in pre- as well as postmenopausal breast cancer patients. The fact that it produces clinical benefits not only in these two groups of patients that differ significantly with respect to plasma estrogen levels but also among patients with very low plasma estrogen levels due to previous hypophysectomy, adrenalectomy or treatment with first/second generation aromatase inhibitors, suggests estrogen deprivation to work independent of pretreatment plasma estrogen levels. Interestingly, in vitro studies have revealed MCF-7 cells to respond to estrogen deprivation by sensitization, causing maximum estradiol stimulation at a concentration 10(-5) to 10(-4) the concentration needed in wild-type cells. While results from recent phase III studies comparing novel aromatase inhibitors and inactivators to conventional therapy have suggested that a more effective hormone ablation may be translated into an improved clinical efficacy, the biochemical rationale for lack of complete cross-resistance between aromatase inhibitors and inactivators or aromatase inhibitors and megestrol acetate remains to be explained. Interestingly, patients becoming resistant to estrogen deprivation may still respond to estrogens administered in pharmacological doses. Future studies are warranted to explore alterations in gene expression and signaling mechanisms in response to different therapies in tumor tissue in vivo.