Traumatic brain injury (TBI) is associated with significant morbidity and mortality. Several studies have demonstrated neuroprotective effects of cannabinoids. The objective of this study was to establish a relationship between the presence of a positive toxicology screen for tetrahydrocannabinol (THC) and mortality after TBI. A 3-year retrospective review of registry data at a Level I center of patients sustaining TBI having a toxicology screen was performed. Pediatric patients (younger than 15 years) and patients with a suspected nonsurvivable injury were excluded. The THC(+) group was compared with the THC(-) group with respect to injury mechanism, severity, disposition, and mortality. Logistic regression was used to determine independent associations with mortality. There were 446 cases meeting all inclusion criteria. The incidence of a positive THC screen was 18.4 per cent (82). Overall mortality was 9.9 per cent (44); however, mortality in the THC(+) group (2.4% [two]) was significantly decreased compared with the THC(-) group (11.5% [42]; P = 0.012). After adjusting for differences between the study cohorts on logistic regression, a THC(+) screen was independently associated with survival after TBI (odds ratio, 0.224; 95% confidence interval, 0.051 to 0.991; P = 0.049). A positive THC screen is associated with decreased mortality in adult patients sustaining TBI.