Background It is essential to clarify the interactions of hormones during the progression of human breast cancer. This study examined the effects of exogenous human leptin on estrogen receptor (ER) alpha and beta in human breast tumor tissue in a nude mouse xenograft model. Methods We created nude mice xenografts of MCF-7 human breast cancer cells, and randomly divided them into an experimental group and a control group. The mice in experimental group were injected subcutaneously around tumors with human leptin, while the control group were injected with the same dose of normal saline. A real-time RT-PCR assay was developed to quantify the mRNA of ER alpha, beta in the tumor tissues. Western blotting analyses were used to assess the relative quantities of the ER alpha, beta proteins. Results Leptin-treated xenografted nude mice were successfully established. The amount of ER alpha mRNA was significantly higher in the leptin group than in the control group (P<0.01), while the amount of ER beta mRNA was significantly lower in the leptin group than in the control group (P<0.01). Western blotting analyses revealed that the ER alpha protein level was significantly higher in the leptin group than in the control group (P<0.01), while the ER beta protein level was significantly lower in the leptin group than in the control group (P<0.01). Conclusions Nude mouse xenograft model can be safely and serviceably treated with human leptin by subcutaneous injections around tumor. ER alpha, beta were both targets of leptin in breast cancer. Leptin can up-regulate the expression of ERa and down-regulate the expression of the ER beta in human breast tumor. Chin Med J 2010;123(3):337-343