BACKGROUND AND PURPOSE: Tumor “size” is used internationally as a surrogate marker for overall survival when following current response assessment protocols (World Health Organization and Response Evaluation Criteria in Solid Tumors). With little evidence of a relationship between tumor “size” and survival in intrinsic brain tumors, this study was undertaken to investigate the predictive value of MR imaging-defined tumor size for survival in patients with recurrent malignant glioma and to compare the different measures of tumor size used in these current response assessment protocols. METHODS: Volumetric, bidimensional, and unidimensional measurements of tumor size were made using baseline contrast-enhanced T1-weighted images of 70 patients with recurrent malignant glioma receiving intravenous chemotherapy. Cox’s proportional hazards model was used to investigate the prognostic importance of tumor size using survival as the end point. Further statistical analysis was undertaken to investigate the relationship between the different measurement techniques. RESULTS: Only the volumetric measurement of tumor size was found to be predictive of survival in recurrent malignant glioma on both univariate and multivariate analysis. Furthermore, analysis demonstrated that the unidimensional and bidimensional measures of tumor were not comparable with the more accurate and direct volumetric measurement. CONCLUSION. Indirect unidimensional and bidimensional measurement techniques do not have a significant association with overall survival or adequately assess tumor size in recurrent malignant glioma. These findings have serious implications about the validity of using current response assessment protocols in therapy trials for recurrent malignant glioma.