Estrogen is a risk factor of breast cancer. Elevated expression of aromatase (estrogen synthase) in breast tissues increases local estradiol concentrations and is associated with breast cancer development, but the causal relationship between aromatase and breast cancer has not been identified. Accumulating data suggest that both estrogen receptor (ER)-dependent and -independent effects are involved in estrogen carcinogenesis. We established a model by expressing aromatase in ERα- MCF-10A human breast epithelial cells to investigate ERα-independent effects of estrogen in the process of malignant transformation. Overexpression of aromatase significantly increased anchorage-independent growth. Parental- or vector-expressing MCF-10A cells did not form colonies under the same conditions. The anchorage-independent growth of MCF-10A(arom) cells can be completely abolished by pre-treatment with the aromatase inhibitor, letrozole. Neither MCF-10A(arom) nor MCF-10A(vector) cells grown in monolayer were affected by short-term exposure to estradiol. Enhanced motility is another characteristic of cellular transformation. Motility of MCF-10A(arom) cells was increased, which could be inhibited by letrozole. Increases in stem cell population in breast cancer tissues are associated with tumor recurrence and metastasis. CD44(high)/CD24(low) is a stem cell marker. We found that CD24 mRNA levels were reduced in MCF-10A(arom) cells compared with those in parental- and vector-transfected cells. By examining individual clones of MCF-10A(arom) with various aromatase activities, we found that the CD24 mRNA levels were inversely correlated with aromatase activity. The ability of MCF-10A(arom) cells to form mammospheres in the absence of serum was increased. Our results suggest that overexpression of aromatase in MCF-10A cells causes malignant transformation. Estrogen metabolite-mediated genotoxicity and induction of a stem cell/progenitor cell population are possible mechanisms. These studies provide additional evidence for ERα-independent mechanism(s) in estrogen carcinogenesis and implicate superiority of aromatase inhibitors to antiestrogens for breast cancer prevention.