Background. We aimed to investigate the relationship between medical decisional capacity (MDC) and Karnofsky Performance Status (KPS) in adults with malignant brain tumors. Methods. Participants were 71 adults with primary (n = 26) or metastatic (n = 45) brain tumors. Testing to determine KPS scores and MDCwas performed as close together as possible for each patient. Participants were administered a standardized measure of medical decision-making capacity (Capacity to Consent to Treatment Instrument [CCTI]) to assess 3 treatment consent abilities (ie, appreciation, reasoning, and understanding). Capacity classifications (ie, capable, marginally capable, and incapable) were established using cut scores previously derived from healthy control CCTI performance. Results. The majority of participants had KPS scores of 90-100 (n = 39), with the remainder divided between KPS scores of 70-80 (n = 26) and 50-60 (n = 6). Comparisons between persons with KPS scores of 90-100 or 70-80 revealed significant differences on the CCTI consent standards of understanding and appreciation. Participants with KPS ratings of 90-100 achieved 46% capable classifications across all CCTI standards, in contrast with 23% of participants with KPS ratings of 70-80, and 0% of participants with KPS ratings of 50-60. Conclusions. A substantial portion of brain-tumor patients with KPS scores reflecting only minimal disability nonetheless demonstrated impairments on standardized measures of MDC. Clinicians working with this adult population should carefully screen for capacity to make clinical treatment decisions regardless of functional/performance status.