Background It is important to identify the multiple sites of leptin activity in obese women with breast cancer.In this study,we examined the effect of exogenous human leptin on heat shock protein 70 (HSP70) expression in MCF-7 human breast cancer cells and in a breast carcinoma xenograft model of nude mice.Methods We cultured MCF-7 human breast cancer cells and established nude mice bearing xenograffs of these cells,and randomly divided them into experimental and control groups.The experimental group was treated with human leptin,while the control group was treated with the same volume of normal saline.A real-time reverse transcriptase-polymerase chain reaction (RT-PCR) assay was developed to quantify the mRNA expression of HSP70 in the MCF-7 human breast cancer cells and in tumor tissues.Western blotting analysis was applied to quantify the protein expression of HSP70 in the MCF-7 cells.Immunohistochemical staining was done to assess the positive rate of HSP70 expression in the tumor tissues.Results Leptin activated HSP70 in a dose-dependent manner in vitro:leptin upregulated significantly the expression of HSP70 at mRNA and protein levels in MCF-7 human breast cancer cells (P ＜0.001).There was no significant difference in expression of HSP70 mRNA in the implanted tumors between the leptin-treated group and the control group (P＞0.05).Immunohistochemical staining revealed no significant difference in tumor HSP70 expression between the leptin-treated group and the control group (P＞0.05).Conclusions A nude mouse xenograft model can be safely and efficiently treated with human leptin by subcutaneous injections around the tumor.HSP70 may be target of leptin in breast cancer.Leptin can significantly upregulate the expression of HSP70 in a dose-dependent manner in vitro.