Breast cancer, the most common malignancy in women, has a known association with the steroid hormone estrogen. Estrogen receptor alpha (ERalpha) plays an important role in the clinical care of breast cancer patients, both as a prognostic factor and as a therapeutic target. Here, we show that a small interfering RNA (siRNA) against ERalpha downregulates ERalpha expression in human MCF-7 and Bcap-37 breast cancer cells, causing a significant decrease in breast cancer cell proliferation. Tumor cells lacking ERalpha expression grew at a much slower rate than did control cells in vitro. Moreover, ERalpha knockdown in breast cancer cells resulted in decreased, even completely abrogated tumor growth in BALB/c nude mice, providing direct evidence for an essential role of ERalpha in breast cancer growth. Our results suggest siRNA-mediated gene silencing of ERalpha may impair tumorigenicity, and even suppress the tumor growth.