The objective of this study was to investigate the diagnostic value of serum tau protein in determining the severity of traumatic brain injury in patients with mild traumatic brain injury (mTBI) and high-risk patients. Adult patients who presented to our emergency department (ED) with mTBI over 1 year were prospectively enrolled. Patients underwent cranial computed tomography (CT) and were subdivided into high- and low-risk groups, according to the probability of resultant intracranial injury. Serum tau levels of 60 patients and 20 healthy volunteers, who served as a control group, were measured. The mean age of the 60 patients (45 males, 15 females) was 32.5 years (range, 15-66 y). Mean Glasgow Coma Scale (GCS) score was 14+/-0.6. CT scans demonstrated intracranial injury in 11 patients (18.3%) and depressed fracture in 4 patients (6.7%). Serum tau levels of patients (188+/-210 pg/mL), compared with those of controls (86+/-48 pg/mL), were relatively higher; however, differences were not statistically significant (P=.445). Also, serum tau levels of high-risk patients (307+/-246 pg/mL) were significantly higher than those of low-risk patients (77+/-61 pg/mL) (P=.001). A total of 48 patients (80%) were accessible for follow-up after 6 months. Postconcussive syndrome was observed in 8 patients, 5 of whom had serum tau protein levels that were higher than those of the other 3 patients. However, no statistically significant difference was observed (P>.05). Investigators of the present study noted that serum tau levels in patients with mTBI were increased. Therefore, it is believed that this biomarker may prove helpful in identifying high-risk patients with mTBI. However, additional studies are needed to establish the diagnostic value of serum tau in detecting traumatic brain injury in patients with mTBI.