Abstract： Background Signal transducer and activator of transcription 3 (STAT3) is usually constitutively activated in a variety of malignancies. It directly contributes to tumorigenesis, invasion, and metastasis. The surgical treatment of breast cancer has made no breakthroughs in terms of treatment effect, in spite of its long history. Current biotherapies bring a note of optimism to breast cancer treatment. To explore the possibility of a siRNA targeted STAT3 blocking treatment for over-activated tumor cells, we evaluated the efficacy of a STAT3 siRNA on human breast cancer cells in vitro and in vivo.Methods Three MCF-7 human breast cancer cell lines were tested: control MCF-7 cells, non-specific siRNA transfected MCF-7 cells and STAT3 siRNA transfected MCF-7 cells. Expression of STAT3 in MCF-7 cells was inhibited by RNA interference (RNAi). The STAT3 mRNA and protein levels were detected by semi-quantity RT-PCR and Western blotting. Cell proliferation and apoptosis were determined by MTT method and flow cytometry. The three groups of MCF-7 cells mentioned above were transplanted subcutanuously into nude mice and their tumorgenic ability observed. The STAT3 mRNA and protein levels of the samples from tumors in different groups were determined by semi-quantity RT-PCR and Western blotting and compared.Results In STAT3 siRNA transfected MCF-7 cells, the expressions (STAT3/p-actin) of STAT3 mRNA (0.327±0.020) and protein (0.153±0.006) were significantly lower than that in control MCF-7 cells (mRNA 1.093±0.018, protein 1.374±0.022) and non-specific siRNA transfected MCF-7 cells (mRNA 1.035±0.050, protein 1.320±0.033) (P <0.05). MTT showed that cell proliferation was significantly reduced and the cell growth inhibition ratio in the STAT3-siRNA group was (44.00±5.10)%, significantly higher than that in non-specific siRNA transfected MCF-7 cells ((16.10±1.05)%, P <0.05). Flow cytometry results showed that more apoptosis was observed in the STAT3-siRNA group. The rate of apoptosis was (14.79±0.22)%, much higher than in control MCF-7 cells (7.06±0.71) and non-specific siRNA transfected MCF-7 cells (8.45±0.43) (P <0.05). The tumor growth in the STAT3 siRNA transfected MCF-7 cells was significantly slower than in the two control groups. On the 22th day after transplantation the tumor weight ((21.40±10.57) mg) and volume ((41.15±12.17) mm3) in the STAT3 siRNA transfected group were significantly lower than in control group (weight (88.60±12.16) mg, volume (118.45±24.68) mm3) and non-specific siRNA transfected group (weight (57.20±21.86) mg, volume (101.36±21.90) mm3 ) (P <0.05). Both the STAT3 mRNA and protein levels in the tumors from the STAT3 siRNA transfected group were significantly lower than in the tumors from the two control groups.Conclusions STAT3 siRNA can effectively silence the STAT3 gene in vitro and in vivo, increase cell apoptosis rate and significantly decrease cell proliferation, which inhibits the growth of breast cancer cell in vitro. Tumor growth of xenograft mice is significantly inhibited. The results obtained in vivo are in consistency with those in vitro. STAT3 may be a novel therapeutic target for breast cancer and RNA interference has potential clinical application.