A small percentage of people with a mild traumatic brain injury (MTBI) report persistent symptoms and problems many months or even years following injury. Preliminary research suggests that people who sustain an injury often underestimate past problems (i.e., "good old days" bias), which can impact their perceived level of current problems and recovery. The purpose of this study was to examine the influence of the good old bays bias on symptom reporting following MTBI. The MTBI sample consisted of 90 referrals to a concussion clinic (mean time from injury to evaluation = 2.1 months, SD = 1.5, range = 0.8-8.1). All were considered temporarily fully disabled from an MTBI and they were receiving financial compensation through the Worker's Compensation system. Patients provided post-injury and pre-injury retrospective ratings on the 16-item British Columbia Post-concussion Symptom Inventory (BC-PSI). Ratings were compared to 177 healthy controls recruited from the community and a local university. Consistent with the good old bays bias, MTBI patients retrospectively endorsed the presence of fewer pre-injury symptoms compared to the control group. Individuals who failed effort testing tended to retrospectively report fewer symptoms pre-injury compared to those patients who passed effort testing. Many MTBI patients report their pre-injury functioning as better than the average person. This can negatively impact their perception of current problems, recovery from injury, and return to work.