Estrogens can play a critical role in the development of breast cancer. Aromatase which catalyzes the formation of aromatic C18 estrogens from C19 androgens is regarded to be responsible for the cancer local production of estrogen. Studies not only from aromatase transfected breast cancer cells, but also transgenic mouse, which overexpressed aromatase, demonstrated that in situ produced estrogen plays more important roles than circulating estrogens in breast tumor promotion and progression. Matrix-metalloproteinases (MMPs) have been implicated in the proeolytic process, which play important roles in the aggressiveness of cancer cells including invasion of adjacent tissue and metastasis to distant sites. Expression of MMP2 and 9 may be stimulated by estrogens in hormonal dependent breast cancers, since tumor aromatase can stimulate breast cancer growth and progression in both an autocrine and a paracrine manner. Theoretically aromatase overexpression, that causes relatively high estrogen concentration in situ, may be positively related to MMP2 and 9 expression, indicate worse prognosis in breast cancers, and maybe insensitive to tamoxifen therapy. In the present study, we studied the expression of aromatase activity and MMPs in human breast carcinoma both in vitro and in vivo. In human breast carcinoma cell lines including MCF-7, MDA-MB-231 and MDA-MB-435, the expression of aromatase levels both in mRNA and protein activity was related to MMP2 and MMP9. In humam breast cancer samples, we demonstrated that aromatase expressions were strongly associated with MMP2 and MMP9 levels. It was interesting to observe that the positive relationship was only present in the ER and/or PR positive patients. This may indicate that both MMP2 and MMP9 were up regulated by estrogen produced by aromatase through ER. So in endocrine therapy, either blocking the ER by tamoxifen or inhibiting the aromatase by aromatase inhibitors for example letrozole, may both inhibit tumor growth and lower the metastatic potential especially in ER positive breast cancer patients by means of down-regulation of MMP2/9.