The objective of this study was to determine if alcohol potentiates the severity of traumatic brain injury (TBI) in motor vehicle crash (MVC) victims, controlling for crash severity characteristics. Prior studies evaluating effects of alcohol on TBI have not accounted for severity of crash. We evaluated severity of head injury by Marshall score [a classification scale of intracranial pathology on head computed axial tomogram (CT)], and blood-alcohol concentration (BAC), while controlling for crash characteristics [traffic accident deformity score (TAD) and belt use]. Marshall scores were determined from initial CT or autopsy reports, by a neurosurgeon, and were categorized into less severe injury (<3) and more severe (> or =3). Logistic regression using this variable as the outcome parameter and crash characteristics, age and BAC as predictors was done, and the odds ratio (OR) and 0.95 confidence interval (0.95 CI) calculated. Fifty-eight patients were analysed: 41% were BAC positive, 30% had a modified Marshall score of > or =3. Patients with positive BAC were 2.1-fold more likely to have a more severe head injury as measured on CT scan by the Marshall scores. We suggest that alcohol potentiates severity of TBI as determined from head CT among MVC victims. Further research will be needed to substantiate this finding as well as to determine its long-term effect on clinical outcome.