The single nucleotide polymorphism SNP309 with a change from T to G in the promoter region of the MDM2 gene is shown to increase the MDM2 protein levels and attenuates the p53 levels and associates with disease progression in several tumours. In this study, the role of the MDM2 SNP309 T>G polymorphism was investigated with regard to the clinical outcome in glioblastoma multiform patients. Genomic DNA was isolated from paraffin-embedded brain tissue samples of 115 patients diagnosed with GBM, some of whom were treated with radiation therapy + temozolomide and the others were treated with only radiation therapy. The MDM2 SNP309 genotype was amplified by PCR, in which two distinct primer pairs were used for each of the alleles. Investigation of genomic DNA of the 115 GBM patients found that 11 (10%) of them were homozygous for TT, 77 (67%) of them were heterozygous for TG, and 27 (23%) of them were homozygous for GG. There was a statistically significant difference between glioblastoma patients and healthy subjects with respect to the distribution of SNP309 genotype (P = 0.03, chi-square test). Our results suggest that in GBM patients MDM2 SNP309 polymorphism homozygote GG and heterozygote TG were significantly associated with increased tumour risk.