Objective: To investigate empirically change in medical decision-making capacity (MDC) in patients with traumatic brain injury (TBI). Design: Longitudinal study comparing control and TBI groups at hospitalization and at 6 months postinjury. Setting: Inpatient brain injury rehabilitation unit. Participants: Twenty healthy controls and 24 patients with moderate to severe TBI. Interventions: Not applicable. Main Outcome Measures: MDC was measured by using the Capacity to Consent to Treatment Instrument (CCTI). The CCTI evaluates performance on a series of 4 accepted consent abilities, or standards: S1 (evidencing/communicating choice), S3 (appreciating consequences), S4 (reasoning about treatment), and S5 (understanding the treatment situation and choices), and 1 experimental standard [S2] (making the reasonable treatment choice when the alternative choice is unreasonable). In addition, TBI patients were assigned 1 of 3 capacity outcomes (capable, marginally capable, incapable) for each standard. Results: At hospitalization, TBI patients performed equivalently with controls on standards S1 and [S2] but significantly below controls on S3 (P<.001), S4 (P<.02), and S5 (P<.001). At 6-month follow-up, TBI patients showed significant within-group improvement on these 3 standards (S3, S4, S5) but continued to fall significantly below controls on S3 (P<.006) and S5 (P<.001). A group by time interaction emerged on S5 (P<.02). The TBI group showed increasing proportions of capable outcomes on all standards over the 6 months. Conclusions: Patients with TBI showed initial impairment and subsequent partial recovery of MDC over a 6-month period. Complex consent abilities of appreciation, reasoning, and understanding were significantly impaired in hospitalized acute TBI patients. At follow-up, TBI patients showed substantial recovery of reasoning and partial recovery of appreciation and understanding consent abilities. The study suggests the importance in the rehabilitation setting of serial evaluations of MDC in patients with TBI. © 2005 by American Congress of Rehabilitation Medicine and the American Academy of Physical Medicine and Rehabilitation.