T-cell based immunotherapy is an attractive approach for the treatment of multiple tumor types including cervical carcinoma. Immunostimulating DNA containing unmethylated cytosine-guanine (CpG) motifs have been successfully used as adjuvants to enhance immune responses to vaccines designed to trigger antitumor T-cell responses. Using a murine model of cervical carcinoma, we report here that repeated administration of synthetic oligodeoxynucleotides bearing CpG motifs (CpG-ODNs) without the need of vaccination into animals bearing large, established tumors resulted in significant antitumor effects. Both tumor regressions and extended survival resulting from CpG-ODN therapy required the participation of CD8+ T cells. On the other hand, CD4+ T cells were not only not required, but also appeared to inhibit the therapeutic effect of CpG-ODN. Tumor regression correlated with increased infiltration of CD8+ T cells into the tumors and with enhanced expression of MHC class I and II antigens by the tumor cells. Together, these results indicate that CpG therapy could be promising as a single agent for the treatment of some tumors such as cervical carcinoma.