OBJECT The influence of alcohol is assumed to reduce consciousness in patients with traumatic brain injury (TBI), but research findings are divergent. The aim of this investigation was to study the effects of different levels of blood alcohol concentration (BAC) on the Glasgow Coma Scale (GCS) scores in patients with moderate and severe TBI and to relate the findings to brain injury severity based on the admission CT scan. METHODS In this cohort study, 265 patients (age range 16–70 years) who were admitted to St. Olavs University Hospital with moderate and severe TBI during a 7-year period were prospectively registered. Of these, 217 patients (82%) had measured BAC. Effects of 4 BAC groups on GCS score were examined with ordinal logistic regression analyses, and the GCS scores were inverted to give an OR > 1. The Rotterdam CT score based on admission CT scan was used to adjust for brain injury severity (best score 1 and worst score 6) by stratifying patients into 2 brain injury severity groups (Rotterdam CT scores of 1–3 and 4–6). RESULTS Of all patients with measured BAC, 91% had intracranial CT findings and 43% had BAC > 0 mg/dl. The median GCS score was lower in the alcohol-positive patients (6.5, interquartile range [IQR] 4–10) than in the alcohol-negative patients (9, IQR 6–13; p < 0.01). No significant differences were found between alcohol-positive and alcohol-negative patients regarding other injury severity variables. Increasing BAC was a significant predictor of lower GCS score in a dose-dependent manner in age-adjusted analyses, with OR 2.7 (range 1.4–5.0) and 3.2 (range 1.5–6.9) for the 2 highest BAC groups (p < 0.01). Subgroup analyses showed an increasing effect of BAC group on GCS scores in patients with Rotterdam CT scores of 1–3: OR 3.1 (range 1.4–6.6) and 6.7 (range 2.7–16.7) for the 2 highest BAC groups (p < 0.01). No such relationship was found in patients with Rotterdam CT scores of 4–6 (p = 0.14–0.75). CONCLUSIONS Influence of alcohol significantly reduced the GCS score in a dose-dependent manner in patients with moderate and severe TBI and with Rotterdam CT scores of 1–3. In patients with Rotterdam CT scores of 4–6, and therefore more CT findings indicating increased intracranial pressure, the brain injury itself seemed to overrun the depressing effect of the alcohol on the CNS. This finding is in agreement with the assumption of many clinicians in the emergency situation.