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Importance: Traumatic injury is a leading cause of hospitalization among people experiencing homelessness. However, hospital course among this population is unknown., Objective: To evaluate whether homelessness was associated with increased morbidity and length of stay (LOS) after hospitalization for traumatic injury and whether associations between homelessness and LOS were moderated by age and/or Injury Severity Score (ISS)., Design, Setting, and Participants: This retrospective cohort study of the American College of Surgeons Trauma Quality Programs (TQP) included patients 18 years or older who were hospitalized after an injury and discharged alive from 787 hospitals in North America from January 1, 2017, to December 31, 2018. People experiencing homelessness were propensity matched to housed patients for hospital, sex, insurance type, comorbidity, injury mechanism type, injury body region, and Glasgow Coma Scale score. Data were analyzed from February 1, 2022, to May 31, 2023., Exposures: People experiencing homelessness were identified using the TQP's alternate home residence variable., Main Outcomes and Measures: Morbidity, hemorrhage control surgery, and intensive care unit (ICU) admission were assessed. Associations between homelessness and LOS (in days) were tested with hierarchical multivariable negative bionomial regression. Moderation effects of age and ISS on the association between homelessness and LOS were evaluated with interaction terms., Results: Of 1 441 982 patients (mean [SD] age, 55.1 [21.1] years; (822 491 [57.0%] men, 619 337 [43.0%] women, and 154 [0.01%] missing), 9065 (0.6%) were people experiencing homelessness. Unmatched people experiencing homelessness demonstrated higher rates of morbidity (221 [2.4%] vs 25 134 [1.8%]; P < .001), hemorrhage control surgery (289 [3.2%] vs 20 331 [1.4%]; P < .001), and ICU admission (2353 [26.0%] vs 307 714 [21.5%]; P < .001) compared with housed patients. The matched cohort comprised 8665 pairs at 378 hospitals. Differences in rates of morbidity, hemorrhage control surgery, and ICU admission between people experiencing homelessness and matched housed patients were not statistically significant. The median unadjusted LOS was 5 (IQR, 3-10) days among people experiencing homelessness and 4 (IQR, 2-8) days among matched housed patients (P < .001). People experiencing homelessness experienced a 22.1% longer adjusted LOS (incident rate ratio [IRR], 1.22 [95% CI, 1.19-1.25]). The greatest increase in adjusted LOS was observed among people experiencing homelessness who were 65 years or older (IRR, 1.42 [95% CI, 1.32-1.54]). People experiencing homelessness with minor injury (ISS, 1-8) had the greatest relative increase in adjusted LOS (IRR, 1.30 [95% CI, 1.25-1.35]) compared with people experiencing homelessness with severe injury (ISS ≥16; IRR, 1.14 [95% CI, 1.09-1.20])., Conclusions and Relevance: The findings of this cohort study suggest that challenges in providing safe discharge to people experiencing homelessness after injury may lead to prolonged LOS. These findings underscore the need to reduce disparities in trauma outcomes and improve hospital resource use among people experiencing homelessness.