Background. Toll-like receptor 4 (TLR4) is a member of a family of pattern recognition receptors that are involved in the host defense against microbial infection. Little research has investigated the link between TLR4 and cancer. We thus addressed the effect of TLR4 in both the host immune system and cancer cells with regard to its effect on breast cancer progression and metastasis.Methods. Adult female Balb/c mice aged 6-8 weeks were divided into three groups. In group 1, 15 each wild-type and TLR4(-/-) mice were inoculated with 4T1 cells; in group 2, wild-type mice were inoculated with 4T1 cells (n = 15), 4T1 cells transduced with TLR4 lentivirus (n = 15) or with control lentivirus (n = 15); and in group 3, 15 TLR4(-/-) mice were inoculated with 4T1 cells transduced with TLR4 lentivirus. Flank tumor volume was measured with calipers during weeks 2-5. Animals were then humanely killed and the number of macroscopic lung nodules counted.Results. There was a significant increase in tumor volume in weeks 2, 3 and 4 after inoculation of TLR4(-/-) mice with 4T1 cells compared with wild-type mice (p < 0.05). The number of metastatic lung nodules was significantly higher in TLR4(-/-) mice (p < 0.05), and survival of tumor-bearing TLR4(-/-) mice was substantially reduced compared with wild-type mice (p = 0.004). Knockdown of TLR4 from the 4T1 cells led to a relative reduction in lung metastasis, although it did not reach statistical significance.Conclusions. TLR4 exerts both a defensive role at the host level and a negative role at the cancer cell level in this murine metastatic breast tumor model. Further evaluation of the role of TLR4 in breast cancer is warranted.