The purpose of this study was to assess the feasibility and effectiveness of an endurance training program in patients who had sustained a traumatic brain injury (TBI) and were in the post-acute recovery phase. Methods: Thirteen patients gave written consent. Inclusion criteria were: ability to ambulate safely on a treadmill at a minimal speed of 1.6kph (1.0mph), no overt cardiovascular disease, ability to follow 2-step commands, and compliance with the expired gas collection apparatus. A modified Balke protocol was used. Speed was gradually increased to the safest possible speed in the first two minutes at 1% incline. Incline was then increased 2% per minute until two of three peak criteria (O 2 plateau, HR >90% age-predicted max, respiratory exchange ratio (RER) >1.15) or ambulation became unsafe. Multivariate comparisons were performed (alpha=0.05). Results: Peak O 2 improved (23.1sd7.6 vs 30.3sd6.9 mL/min/kg) (p<0.01). Peak HR and RER did not change. Submaximal ventilatory equivalents decreased. Conclusions: Despite the cognitive and behavioral challenges, patients were found to be compliant with properly prescribed aerobic exercise. Further randomized controlled trials are warranted in this population. TBI is a chronic disease and increased physical activity should be encouraged early in the rehabilitation process.