BACKGROUND As emergency care becomes increasingly regionalized, systems planners must determine how system expansion impacts existing system assets. We hypothesized that accrediting new Level II and III trauma centers impacted the trauma patient census and severity at a nearby Level I trauma center and estimated the magnitude of the impact. METHODS We conducted an interrupted time series analysis using monthly patient counts during the past 10 years for five trauma centers located near one another in Pennsylvania. The Level I center (TC-A) operated for the entire period. A Level II center 39 miles away was accredited after 70 months (TC-B), one Level III center 46 miles away was accredited after 95 months but lost accreditation after 11 months (TC-C), and two other Level III centers 40 miles and 45 miles away were accredited after 107 months (TC-D and TC-E). RESULTS Monthly patient volume at the Level I center, which increased gradually over the study and summed to 25,120 patients, decreased by 10.8% (p < 0.05) when TC-B was accredited and decreased by an additional 12.9% (p < 0.05) when TC-D and TC-E were accredited simultaneously. No change stemmed from temporarily accrediting TC-C. Ultimately, the Level I center treated 1,903 fewer patients than expected over 51 months, an 11.9% volume reduction, and patient severity remained consistent but mortality decreased. CONCLUSIONS Accrediting Level II and Level III trauma centers reduced patient volume and reduced overall mortality at a nearby Level I center. Strategic planning of statewide trauma systems can help balance rapid access to care with maintenance of adequate annual patient volumes of critically injured patients. METHODS Epidemiologic study, Level IV.