Vorinostat (suberoylanilide hydroxamic acid), a histone deacetylase inhibitor, is currently undergoing clinical evaluation as therapy for cancer. We investigated the effects of vorinostat on tumor cell radiosensitivity in a breast cancer brain metastasis model using MDA-MB-231-BR cells. In vitro radiosensitivity was evaluated using clonogenic assay. Cell cycle distribution and apoptosis was measured using flow cytometry. DNA damage and repair was evaluated using gammaH2AX. Mitotic catastrophe was measured by immunostaining. Growth delay and intracranial xenograft models were used to evaluate the in vivo tumor radiosensitivity. Cells exposed to vorinostat for 16 hours before and maintained in the medium after irradiation had an increase in radiosensitivity with a dose enhancement factor of 1.57. gammaH2AX, as an indicator of double-strand breaks, had significantly more foci per cell in the vorinostat plus irradiation group. Mitotic catastrophe, measured at 72 hours, was significantly increased in cells receiving vorinostat plus irradiation. Irradiation of s.c. MDA-MB-231-BR tumors in mice treated with vorinostat resulted in an increase in radiation-induced tumor growth delay. Most importantly, animals with intracranial tumor implants lived the longest after combination treatment. These results indicate that vorinostat enhances tumor cell radiosensitivity in vitro and in vivo. There was a greater than additive improvement in survival in our intracranial model. Combining vorinostat with radiation may be a potential treatment option for patients with breast cancer who develop brain metastases.