The aim of this study was to analyze the frequency, time of onset, severity, and impact of hypernatremia in patients with severe TBI on the length and outcome of treatment treated in the Intensive Care Unit. Patients and methods.The respondent was a patient hospitalized in the ICU Clinic of Anesthesiology, Intensive Care Medicine and Pain Treatment Clinical Hospital center Rijeka during 2018, 2019 and 2020 with severe TBI. The analysis was done retrospectively. Results. Out of a total of 47 patients, 13 developed hypernatremia. Most patients developed it on the third day of treatment. Of the patients who developed hypernatremia, 61.5% died, the mean GCS was 5.31 (SD 3,093), the mean age was 52.00 years (SD 22,004), and it lasted an average of 17.38 days. Conclusion. Patients with hypernatremia had, on average, older age and poorer GCS at admission. The duration of care was shorter on average, most likely due to the large number of deaths who died most often between the 3rd and 7th day of care. They also had worse outcomes than patients without hypernatremia. None of the patients who had hypernatremia recovered, and the surviving patients were discharged with or without recovery from a neurological condition, minimal recovery, or with some form of disability. Hypernatremia has been shown to be a predictor of poorer outcome in patients with severe TBI.