The association between increased signs and symptoms and poorer survival in glioblastoma is well recognized and confirmed in virtually every clinical trial and patient series. Measurement and consideration of performance status is, therefore, vital when counseling patients regarding their expected survival or evaluating the results of clinical trials. Unfortunately, measures of patient function in clinical trials have remained quite crude, in stark contrast to the rapid advances seen in the pathological characterization of brain tumors. Recently, clinical investigators have begun to examine the subcomponents of performance status in more detail, revealing that objective measures of cognition are significantly associated with patient survival in both newly diagnosed and recurrent glioblastoma. Furthermore, cognitive function does not appear to be a simple proxy for performance status but rather an independent predictor of survival, even within patient groups defined by currently available clinical prognostic systems. Therefore, objective measures of cognition must be evaluated for inclusion in future prognostic models and the simple addition of new tumor biomarkers to the current clinical prognostic models will likely prove insufficient. In order for the field of neuro-oncology to move forward in this regard, evaluation of cognition must become a routine part of future clinical trials, and the data must be recorded and analyzed with the same diligence as other trial end points.