Background There is a significant association between obesity and breast cancer, which is possibly due to the expression of leptin. Therefore, it is important to clarify the role of leptin/ObR (leptin receptor) signaling during the progression of human breast cancer. Methods Nude mice with xenografts of MCF-7 human breast cancer cells were administered recombinant human leptin subcutaneous via injection around the tumor site. Mice in the experimental group were intratumorally injected with ObR-RNAi-lentivirus, while negative control group mice were injected with the same dose of negative-lentivirus. Tumor size was blindly measured every other day, and mRNA and protein expression levels of ObR, estrogen receptor alpha (ER alpha), and vascular endothelial growth factor (VEGF) for each group were determined. Results Knockdown of ObR-treated xenografted nude mice with a high leptin microenvironment was successfully established. Local injection of ObR-RNAi-lentivirus significantly suppressed the established tumor growth in nude mice. ObR level was significantly lower in the experimental group than in the negative control group, while the amounts of ER alpha and VEGF expression were significantly lower in the leptin group than in the control group (P<0.01 for all). Conclusions Inhibition of leptin/ObR signaling is essential to breast cancer proliferation and possible crosstalk between ObR and ER alpha, and VEGF, and may lead to novel therapeutic treatments aiming at targeting ObR in breast cancers. Chin Med J 2012;125(9):1563-1570