Herpes simplex virus-thymidine kinase (HS-tk) gene therapy with ganciclovir (GCV) treatment has been reported to inhibit the tumor growth, which is applied to the gene therapy targeted to the malignant brain tumor. To suppress the tumor growth completely, the authors designed the HS-tk gene therapy in combination with granulocyte macrophage-colony stimulating factor (GMCSF) gene using the hepatic metastatic model of murine colon cancer. The transduction of the HS-tk gene in combination with the GMCSF gene, followed by GCV, showed a complete inhibition of hepatic metastases of murine colon cancer, which was significantly superior to that of HS-tk gene alone. The growth of cancer cells transduced with both HS-tk and GMCSF genes was inhibited in vitro, and long-lasting antitumor immunity after hepatic metastasis of cancer cells transduced with both HS-tk and GMCSF genes was acquired. It is suggested that HS-tk gene therapy in combination with the GMCSF gene is effective for the complete inhibition of hepatic metastasis of murine colon cancer.