Purpose: Breast cancer patients with brain metastasis have a dismal prognosis. We determined the ability of immunostimulatory CpG oligodeoxynucleotides (QDN) to treat or prevent brain metastasis in a mouse model. Experimental Design: Mice bearing orthotopic breast carcinoma with or without concurrent i.c. tumors were treated by injections of CpG QDN at the primary tumor. Immunologic memory was tested by tumor rechallenge and immune responses were assessed by flow cytometry, delayed-type hypersensitivity, and CTL assays. Results: Qrthotopic tumors regressed in treated mice regardless of whether concurrent i.c. disease was present. In mice bearing only orthotopic tumors, CpG QDN rendered 50% tumor-free and they rejected tumor rechallenge in breast and brain. In mice with concurrent i.c. disease, there was no difference in brain tumor growth compared with saline controls, despite regression of the primary tumor. Flow cytometry revealed that treated mice that died from i.c. disease exhibited a significant increase in brain-infiltrating T and natural killer cells relative to saline controls. CTLs from these mice were able to kill tumor in vitro and extend survival of naive mice bearing less-established brain tumors by adoptive transfer. Conclusions: The lack of survival benefit in mice with appreciable brain metastasis was not explained by a deficit in lymphocyte trafficking or function because CTLs from these mice killed tumor and inhibited microscopic brain metastasis by adoptive transfer. These results indicate that CpG QDN might be beneficial as a preventative adjuvant to initial therapy preceding brain metastasis or to inhibit progression of microscopic brain metastases.