The aim of this study was to identify if there was an association between physical activity levels and fatigue following Mild Traumatic Brain Injury (MTBI). A secondary aim was to establish if either of these two variables, physical activity and fatigue, was associated with the functional outcome of achieving a return to work. Participants were recruited through the Accident Compensation Corporation (ACC) research unit via an internal service code, which identified potential participants as having sustained a MTBI within the last six months, currently receiving rehabilitation services and having had a referral for assessment at a MTBI clinic. A total of 216 individuals were subsequently sent a questionnaire via post. The questionnaire pack included: • The New Zealand Physical Activities Questionnaire – Short Form (NZPAQ-SF), to measure self-reported physical activity levels. • The Checklist Individual Strength (CIS) Questionnaire, to measure self-reported fatigue levels. • Demographic questions including; age, gender, ethnicity, date of injury, where the injury occurred, how the injury occurred, previous brain injuries, history of mood disorders and any outcome of a return to work. Following a response rate of 47% (n = 101) data analysis identified a significant negative association between fatigue and physical activity (r s = 0.38, p<0.01); indicating that higher levels of fatigue were associated with lower levels of physical activity following a MTBI. A significant association was also identified between fatigue and the likelihood of being at work (OR 1.03, p<0.01), indicating that higher levels of fatigue were associated with a lower chance that an individual will be at work following MTBI. A non-significant association (OR 0.96, p = 0.12), was identified between physical activity and the likelihood of an individual being at work. Further, 30.7% of the population reported severe fatigue based on their scores on the CIS. Despite this, overall activity levels were comparable to a general non-MTBI population. This research has extended understanding of the complex relationship between fatigue, exercise and work post MTBI. Further research is needed to identify the direction of the relationship between these variables and thus the optimum focus for interventions. However, the findings here have indicated that the advice clinicians give about rest and activity potentially plays a crucial role in recovery and adaptation after MTBI. In particular, the role improved activity management might have in ameliorating fatigue post MTBI, must be identified if return to work rates for this population are to improve.