Introduction. Traumatic brain injury (TBI) is a defect in the brain function resulting from the action of external factors. The condition ranges from transient shifts in cellular ionic concentrations to total structural damage; the clinical symptoms can vary from brief confusion to death. The current classification system, based on the Glasgow Coma Scale (GCS), divides TBI into mild (GCS 14 to 15), moderate (GCS 9 to 13), and severe TBI (GCS 3 to 8). The leading causes of head injury in the population can be falls, motor vehicle collisions, blasts, and bullet injuries. The ultimate survival and neurologic outcome of the head trauma patient depend on the extent of TBI occurring at the time of injury. The aim of the study is to assess the factors associated with brain injury and their effect on its severity. Method. A cross sectional, retrospective study including 469 adult patients with head injury was carried out in the emergency department of Baghdad teaching hospital between 1 October 2016 - 30 October 2017. Data of all the patients were entered and analyzed using the statistical package for social sciences (SPSS) software for Windows, version 24. The significance of correlation was assessed using a Chi-square test. Level of significance was set at ≤ 0.05. Final findings were presented in tables with an explanatory paragraph for each table using the MS. Office (Word 2013) for Windows. Results. A total number of 469 patients were enrolled in this study with a mean age of 42.6 ± 13.7 years. The vast majority of the patients were males (383/469), which represented 81.7%. The male to female ratio was 4.45 to 1. The distribution of the patients according to their traumatic brain injury was 241 patients (51.4%) with mild, 99 (21.1%) with moderate and 129 (27.5%) with severe TBI. Associated injuries among the studied group were facial injuries in 133 (28.4%), skull fracture in 150 (32%), and thoracolumbar fracture in 51 (10.9%). The associated injuries were more frequent in the extremities - 112/469 (23.9%), combined injuries in 112 (23.9%), chest and abdomen injuries were reported in only 6% and 6.8%, respectively. The causes of injuries in the studied group were road traffic accidents (RTA), the most frequent mechanism - 37.1% of the cases, followed by fall from height (FFH) (23.9%), blast injury (16.8%), bullet injury (13%), while other mechanisms represented only (9.2%). There is a significant association reported with gender, where severe injuries were more frequent among males than females (P=0.014). All associated injuries were significantly associated with severe traumatic brain injury (P<0.05), also bullet injury was significantly associated with severe traumatic brain injury, followed by blast injury (P<0.001). Severe traumatic brain injury was significantly associated with the presence of clinical and radiographic findings (P<0.001). Conclusion. The severity of TBI is significantly related to the type of the associated injury, mechanism of injury, clinical and radiological findings, and to the male gender, while it is not dependent on the age of the patient.