We report that 100% mice survival after tumor challenge is achieved with cytokine-engineered cells employing nonviral lipoplexes and without using viral vectors. We describe this effect with cytokine-secreting tumor cell vaccines, based on cell clones or fresh transfected cells. Tumor cells were transfected with murine granulocyte-macrophage colony-stimulating factor (GM-CSF) or IL-4 plasmids employing the cationic lipid DOTAP, were irradiated (150 Gy) and kept frozen until use. The transfection efficacy was analyzed by qRT-PCR and flow cytometry. Vaccination induced potent antitumor rejection, resulting in 100% mice survival. Furthermore, the antitumor immunity was long lasting, since a two-fold survival delay was observed in mice after tumor rechallenge (6 months later). While cell clones secreting GM-CSF were the most effective in wild-type tumor cell rejection, little or no effect was observed with clones secreting IL-4. We found similar antitumor efficacy employing fresh transfected cells by nonviral procedures, demonstrating that cells genetically modified by nonviral vectors (both clones and fresh transfected cells) are a safe and efficient tool for antitumor vaccines. These vaccines allow us to achieve the highest antitumor efficacy based on nonviral gene therapy techniques. In addition, the vaccination success with fresh transfected cells simplifies the procedure and provides new insights into the clinical application of nonviral gene therapy procedures.