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To evaluate the association of trauma center volume change over time with mortality.Regionalization of trauma systems assumes a volume–outcome relationship for severe injury. Whereas this has been shown for cross-sectional volume, it is unclear whether volume changes over time translate into predictable outcome changes.Retrospective cohort study of severely injured (injury severity score >15) patients from the National Trauma Databank 2000 to 2012. A center-level standardized mortality ratio (SMR) was constructed (ratio of observed to expected deaths). Expected mortality was obtained from multilevel logistic regression model, adjusting for demographics, mechanism, vital signs, and injury severity. Center-level percent volume change was assessed across early (2000–2006) and late (2007–2012) periods. Longitudinal panel modeling evaluated association between annual SMR change and volume change over preceding years.There were 839,809 patients included from 287 centers. Each 1% increase in volume was associated with 73% increased odds of improving SMR over time [odds ratio (OR) 1.73; 95% confidence interval (CI) 1.03–2.91; P = 0.03]. Each 1% decrease in volume was associated with 2-fold increase in odds of worsening SMR over time (OR 2.14; 95% CI 1.07–4.26, P = 0.03). Significant improvement in the SMR emerged after 3 or more preceding years of increasing volume (SMR change −0.008; 95% CI −0.015, −0.002; P = 0.01). This benefit occurred only in centers that were level I or II verified.Increasing volume was associated with improving outcomes, whereas decreasing volume was associated with worsening outcomes. High-level trauma center infrastructure seems to facilitate the volume–outcome relationship. The trauma center designation process should consider volume changes in the overall system.