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Objective. It is now accepted that the addition of temozolomide to radiotherapy in the treatment of patients with newly diagnosed glioblastoma multiforme (GBM) significantly improves survival. In 2008, a subanalysis of the original study data was performed, and an online 'GBM Calculator' was made available on the European Organisation for Research and Treatment of Cancer (EORTC) website allowing users to estimate patients' survival outcomes. We tested this calculator against actual local survival data to validate its use in our patients. Materials and methods. Prospectively collected clinical data were analysed on 105 consecutive patients receiving concurrent chemoradiotherapy following surgical treatment of GBM between December 2004 and February 2009. Using the EORTC online calculator, survival outcomes were generated for these patients and compared with their actual survival. Results. The median overall survival for the entire cohort was 15.3 months (range 2.8-50.5 months), with 1-year and 2-year overall survival of 65.7% and 19%, respectively. This is in comparison to the median overall predictive survival of 21.3 months, with 1-year and 2-year survival of 95% and 39.5%, respectively. Case by case analysis also showed that the survival was overestimated in nearly 80% of patients. Subgroup analyses showed similar overestimation of patients' survival, except calculator Model 3 which utilised MGMT status. Conclusion. Use of the EORTC GBM prognostic calculator would have overestimated the survival of the majority of our patients with GBM. Uncertainty exists as to the cause of overestimation in the cohort although local socioeconomic factors might play a role. The different calculator models yielded different outcomes and the 'best' predictor of survival for the cohort under study utilised the tumour MGMT status. We would strongly encourage similar local studies of validity testing prior to employing the online prognostic calculator for other population groups. [ABSTRACT FROM AUTHOR]