Purpose: To investigate research consent capacity (RCC) in patients with malignant gliomas (MGs) and identify cognitive abilities and clinical factors associated with RCC in MG. Patients and Methods: Participants were 22 healthy controls and 26 patients with diagnosed and histologically verified MG. All patients with MG were receiving various combinations of treatment (surgery, radiation, chemotherapy, medication). Both groups were administered a neurocognitive test battery and a standardized RCC measure (Capacity to Consent to Research Instrument [CCRI]). Capacity performance was evaluated across four core consent abilities (choice, appreciation, reasoning, understanding), and categorical capacity impairment ratings (no impairment, mild/moderate impairment, severe impairment) were also identified for individual patients with MG. Stepwise regression analyses identified cognitive predictors of CCRI performance in the MG group. Results: The MG patient group performed significantly below the control group on the three clinically relevant consent standards (appreciation, reasoning, and understanding). Patients with MG performed equivalently to controls in evidencing a simple research participation choice. Approximately one third of patients with MG showed compromised impairment ratings (mild/moderate impairment or severe impairment) on the three consent abilities. Cognitive measures of phonemic and semantic word fluency predicted performance on the consent standards. Steroid treatment and anticonvulsant use were related to poorer CCRI performance. Conclusion: A substantial portion of patients with MG after diagnosis show impairments in research consent capacity. Word fluency measures reflecting expressive language and executive function abilities are strongly associated with these consent impairments. This study highlights the importance of careful attention to consent issues when enrolling patients with MG in clinical trials and other research studies. © 2010 by American Society of Clinical Oncology.