Radiotherapy is the standard treatment for glioblastoma. Here, we assessed the radiosensitivity of 12 human malignant glioma cell lines in vitro and correlated these data with irradiation-induced cell cycle changes, chemosensitivity profiles and BCL-2 family protein expression. Irradiation at 3 Gy failed to cause major cell cycle perturbations. Radioresistance was associated with collateral sensitivity to the topoisomerase II inhibitors, teniposide and doxorubicin. High levels of BCL-XL and low levels of BAX were independently linked to radioresistance. Ectopic expression of a BAX transgene induced radiosensitization in the LN-18 cell line. Thus, BCL-2 family protein expression modulates radiosensitivity in human glioma cells and targeted alterations in BCL-2 family protein expression are a promising strategy to improve the therapeutic efficacy of radiotherapy for gliomas.