BACKGROUND: Previous studies of the center volume-outcomes relationship for severe trauma care have yielded conflicting findings regarding the presence or nature of such a relationship. Few studies have confined their analysis to Level I centers. METHODS: We performed a retrospective analysis of severely injured adults treated from 2001 through 2006 in United States Level I trauma centers using data from the National Trauma Data Bank version 7.1. The post-injury in-hospital mortality rates for patients treated at high- or medium-volume Level I trauma centers were compared with the rates for patients treated at low-volume Level I centers before and after adjustment for patient demographic and injury characteristics. Subgroup comparisons were performed for those Level I centers with and without American College of Surgeons (ACS) verification of Level I designation. RESULTS: Overall, medium-volume Level I trauma centers had significantly lower mortality than low-volume centers (14.3% versus 15.6%), both before and after adjustment for patient demographic and injury characteristics. Of those trauma centers without ACS verification of Level I designation, high-volume centers had significantly greater mortality than low-volume centers. CONCLUSIONS: Our findings support the current utilization by the American College of Surgeons of minimum annual volume requirements for the verification of Level I trauma center designation, and suggest that the presence of such verification may enable Level I centers to effectively manage high volume of severely injured adult patients.