Background: Traumatic brain injury (TBI) is a leading cause of death among patients in developed countries. The patients' prognosis depends on the trauma-induced primary damage as well as the secondary brain damage, including electrolyte disturbances. Therefore, prevention, diagnosis, and timely treatment lead to better prognosis. Herein, the aim is to prognosticate about the mortality in patients with TBI through serum osmolarity at admission. Materials and Methods: In this cross-sectional study, 141 patients with TBI were assigned through convenience sampling. The level of serum osmolarity was examined once the patients were admitted to emergency department and later, the outcome was recorded. Finally, we analyzed the relationship between osmolarity level and patient outcome in age groups. Results: The mean serum osmolarity in the age group of under 18 years, 18 to 60 years, and more than 60 years was equal to 295.3 ± 10.02 mOsm/L, 297.2 ± 6.5 mOsm/L, and 301.6 ± 7.6 mOsm/L, respectively (P-value