Background Oncolytic herpes simplex virus (HSV) vectors can be used for cancer therapy as direct cytotoxic agents, inducers of anti-tumor immune responses, and as expressers of anti-cancer genes. In this study, the efficacy of HSV vectors, G47Δ and NV1023 were examined for the treatment of the human breast cancer.Methods Human breast cancer MDA-MB-435 cells were cultured or implanted subcutaneously in BALB/c nude mice. The cells or tumors were inoculated with G47Δ or NV1023, and cell killing or inhibition of tumor growth determined. Both viruses contained the LacZ gene and expression in infected cells was detected with X-gal histochemistry. Results G47Δ and NV1023 were highly cytotoxic to MDA-MB-435 cells in vitro at very low multiplicities of infection. X-gal staining of infected tumor cells in vitro and in vivo illustrated the replication and spread of both viruses. G47Δ and NV1023 inoculation inhibited tumor growth and prolonged mouse survival. Both vectors behaved similarly.Conclusions Oncolytic HSV vectors, G47Δ and NV1023, were extremely effective at killing human breast cancer cells in vitro and in tumor xenografts in vivo. This novel form of cancer therapy warrants further investigation and consideration of clinical application.