Background The prevalence and impact of prehospital neurologic deterioration (PhND) in patients with traumatic brain injury (TBI) have not been investigated. We aimed to determine the prevalence of PhND during emergency medical service (EMS) transportation among patients with TBI and its impact on patient's outcome. Methods We used the National Trauma Data Bank, using data files from 2009 to 2010 to identify patients with TBI through International Classification of Diseases, Ninth Revision, Clinical Modification diagnosis codes. The initial Glasgow Coma Scale (GCS) score ascertained at the scene by EMS was compared with the subsequent GCS score evaluation in the emergency department (ED) to identify neurologic deterioration (defined as a decrease in GCS of ≥ 2 points). Patients' demographics, initial injury severity score (ISS), admission GCS score, and hospital outcome were compared between patients with PhND and patients without neurologic deterioration. Results A total of 257 127 patients with TBI were identified. Among patients with TBI, 22 254 patients had PhND, which comprised 9% of all patients with TBI. The mean of GCS score decrease during EMS transport was 5 points (± 3). Patients without PhND tended to have higher GCS recorded by EMS (median, 15 vs 12; P <.0001). Patients with TBI who had PhND had significantly higher hospital length of stay and intensive care unit days after adjusting for baseline characteristics and EMS GCS score, EMS transport time, type of injury, presence of intracranial hemorrhages, and ED ISS (P <.0001). These patients had higher rate of in-hospital mortality after adjusting for the same variables (odds ratio, 2.30; 95% confidence interval, 2.18-2.41). Conclusion Prehospital neurologic deterioration occurs in 9% of patients with TBI. It is more prevalent in men and associated with lower EMS GCS level and higher ED ISS. Prehospital neurologic deterioration is an independent predictor of worse hospital outcome and higher resource use in patients with TBI.