Gene therapy with Herpes Simplex Virus thymidine kinase gene (HSV-tk) is effective in various tumor models in vitro and in vivo. We compared the efficacy of the HSV-tk gene therapy in vitro and in vivo in MCF-7 and MCF7-ras cells which form tumor in athymic mice. After viral infection, cells were treated with GCV (Ganciclovir) and live cells were counted. The in vitro treatment significantly inhibited cell growth but did not induce early and late apoptosis, measured, respectively, by annexin or by propidium iodide staining and a significant cell death. The HSV-tk/GCV treatment of MCF7-ras tumor in athymic mice showed a significant inhibition of tumor development until 60 days post-treatment. Some mice showed a complete tumor eradication without tumor regrowth after the end of treatment. In conclusion, we demonstrated that the HSV-tk/GCV system is not very efficient in vitro, but very efficient in vivo in our animal breast cancer model.