Trauma systems have been shown to decrease injury-related mortality; however, their development has been slow often requiring legislative codification. The purpose of this study was to evaluate the impact of a voluntary regional trauma system on outcomes at a Level I trauma center. We conducted a retrospective cohort study in an American College of Surgeons-verified Level I trauma center including all patients admitted to a Level I trauma center during the periods April 1995 through March 1996 (T-1) and April 1997 through March 1998 (T-2). Our main outcome measures were in-hospital mortality, hospital length of stay, cost of care Compared with T-1 patients T-2 patients had lower mortality (odds ratio 0.48, 95% confidence interval 0.32-0.71). A similar decline in mortality was observed for the entire six-county region compared with the remainder of the state. Among the most severely injured patients (Injury Severity Score > or = 16) T-2 patients had a shorter length of stay (16.5 vs 19.5 days; P < 0.05) and lower mean cost of care ($29,795 vs $34,983; P < 0.05). A voluntary trauma system can be implemented without the need for legislative mandate. After system implementation patient and financial outcomes were improved at an individual Level I trauma center.