Glioblastoma multiforme (GBM) is the most common primary brain tumour and extirpation followed by radio- and chemotherapy has had minimal impact on the median survival of patients which is still less than one year. Hence, a novel therapeutic modality is required if the survival of patients with this disease is to be improved. ATM, mutated in the human genetic disorder ataxia-telangiectasia (A-T), plays a central role in the response to DNA double strand breaks and patients with this disorder are characterised by extreme sensitivity to radiation, increased risk of cancer and neurodegeneration. Thus, ATM represents a potential target for radiosensitization of brain tumour cells. A safe, non-replicating lentivirus is used to abrogate ATM in GBM through the antisense and RNAi approaches for radiosensitization. With either techniques, ATM protein was reduced by >90% and there was a 3‑fold sensitization of GBM cells to radiation. ATM protein activation as well as ATM pS1981 foci formation were defective and downstream signalling determined by Ser15 phosphorylation on p53 was reduced. Success in the approaches provides a novel and exciting strategy for the treatment of GBM and thus improving the survival of patients with these tumours.