To prevent serious complications and facilitate efficient and effective management of patients admitted to the emergency department or intensive care settings, it is extremely important to differentiate, quickly, between elevated concentrations of alcohol in the blood and compromised neurological status due to brain injury or other pathology. This research analyzed the relationship between blood alcohol concentrations (BACs) estimated from venous blood samples and those estimated from breath samples that were obtained using the Alco-Sensor III device with an attached tube for passive nasal breath sampling. Blood and breath samples, as well as brief medical histories and demographic and environmental data, were obtained and recorded for 35 adult trauma patients admitted to two major urban emergency departments. Passively expired nasal breath provided an excellent estimate of BAC measured from venous blood (range, 0 to 0.32) as evidenced in the extremely high regression coefficient (r = 0.99; slope = 1.22; p less than 0.0001). BAC assessment and monitoring, through the application of passive nasal breath sampling, provides a means of rapidly estimating BAC, and thus can facilitate diagnosis and the initiation of appropriate management and treatment.