Objective: To investigate medical decision-making capacity (MDC) in patients with acute traumatic brain injury (TBI) across a range of injury severity. Methods: We evaluated MDC cross-sectionally 1 month after injury in 40 healthy controls and 86 patients with TBI stratified by injury severity (28 mild [mTBI], 15 complicated mild [cmTBI], 43 moderate/severe [msevTBI]). We compared group performance on the Capacity to Consent to Treatment Instrument and its 5 consent standards (expressing choice, reasonable choice, appreciation, reasoning, understanding). Capacity impairment ratings (no impairment, mild/moderate impairment, severe impairment) on the consent standards were also assigned to each participant with TBI using cut scores referenced to control performance. Results: One month after injury, the mTBI group performed equivalently to controls on all consent standards. In contrast, the cmTBI group was impaired relative to controls on the understanding standard. No differences emerged between the mTBI and cmTBI groups. The msevTBI group was impaired on almost all standards relative to both control and mTBI groups, and on the understanding standard relative to the cmTBI group. Capacity compromise (mild/moderate or severe impairment ratings) on the 3 clinically complex standards (understanding, reasoning, appreciation) occurred in 10%-30% of patients with mTBI, 50% of patients with cmTBI, and 50%-80% of patients with msevTBI. Conclusions: One month following injury, MDC is largely intact in patients with mTBI, but is impaired in patients with cmTBI and msevTBI. ImpairedMDCis prevalent in acute TBI and is strongly related to injury severity. Copyright © 2012 by AAN Enterprises, Inc.