ABSTRACT: Importance:Controversy remains as to whether advanced life support (ALS) or basic life support (BLS) is superior for critically ill and injured patients, including out-of-hospital cardiac arrest (OHCA) and major trauma, in the prehospital setting. Objective:To assess whether prehospital ALS should be provided for traumatic OHCA and who should perform it. Design, Setting, and Participants:Japanese government-managed nationwide population-based registry data of patients with OHCA transported to an emergency hospital were analyzed. Patients who experienced traumatic OHCA following a traffic collision from 2013 to 2014 were included. Patients provided prehospital ALS by a physician were compared with both patients provided ALS by emergency medical service (EMS) personnel and patients with only BLS. The data were analyzed on May 1, 2017. Exposures:Advanced life support by physician, ALS by EMS personnel, or BLS only. Main Outcomes and Measures:The primary outcome was 1-month survival. The secondary outcomes were prehospital return of spontaneous circulation and favorable neurologic outcomes with the Glasgow-Pittsburgh cerebral performance category score of 1 or 2. Results:A total of 4382 patients were included (mean [SD] age, 57.5 [22.2] years; 67.9% male); 828 (18.9%) received prehospital ALS by physician, 1591 (36.3%) received prehospital ALS by EMS personnel, and 1963 (44.8%) received BLS only. Among these patients, 96 (2.2%) survived 1 month after OHCA, including 26 of 828 (3.1%) for ALS by physician, 25 of 1591 (1.6%) for ALS by EMS personnel, and 45 of 1963 (2.3%) for BLS. After adjusting for potential confounders using multivariable logistic regression, ALS by physician was significantly associated with higher odds for 1-month survival compared with both ALS by EMS personnel and BLS (adjusted OR, 2.13; 95% CI, 1.20-3.78; and adjusted OR, 1.94; 95% CI, 1.14-3.25; respectively), whereas there was no significant difference between ALS by EMS personnel and BLS (adjusted OR, 0.91; 95% CI, 0.54-1.51). A propensity score-matched analysis in the ALS cohort showed that ALS by physician was associated with increased chance of 1-month survival compared with ALS by EMS personnel (risk ratio, 2.00; 95% CI, 1.01-3.97; P = .04). This association was consistent across a variety of sensitivity analyses. Conclusions and Relevance:In traumatic OHCA, ALS by physician was associated with increased chance of 1-month survival compared with both ALS by EMS personnel and BLS.