We report the use of plasmid DNA-mediated combination gene therapy for tumor-bearing mice using in vivo electroporation, also called electro-gene therapy (EGT), that resulted in uncomplicated and complete cures in more than 90% of the mice. Subcutaneously inoculated CT26 tumors in syngeneic BALB/c mice were subjected to repeated EGT treatments consisting of intratumoral coinjection of naked plasmids encoding the cytokine interleukin-12 (IL-12) (p35 and p40 subunits) and the suicide gene herpes simplex virus thymidine kinase (HSV-tk), followed by in vivo electroporation. The early anti-tumor effect was always stronger, and the rate of cure, as seen in the long-term follow-up, was always greater in the groups treated with combination EGT than in those treated with IL-12 or HSV-tk EGT alone. Systemic levels of IL-12 and IFN-gamma increased in both combination and IL-12-alone EGT-treated groups. Moreover, combination EGT for established subcutaneous tumors strongly reduced hematogenous lung metastases and increased survival time when live CT26 tumor cells were injected through the tail vein. Limited experiments on C57/B16 mice with murine melanoma also showed very similar trends. These results suggest that this simple and safe method of plasm id-mediated combination EGT may provide a potentially effective gene therapy for cancer.