Background: Many patient population groups are not proportionally represented in clinical trials, including patients of color, at age extremes, or with comorbidities. It is unclear how treatment outcomes may differ for these patients compared to those well represented in trials. Methods: This retrospective cohort study included women diagnosed with early-stage (I-III) breast cancer (EBC) between 2005-2015 in the CancerLinQ Discovery electronic medical record-based dataset. Patients with comorbidities or concurrent cancer were considered unrepresented in clinical trials. Non-White patients and/or those aged <45 or ≥70 years were considered underrepresented. Patients who were White and aged 45-69 were considered well represented. Overall and EBC subtype-stratified Cox proportional hazards models estimated hazard ratios (HR) and 95% confidence intervals (CI) for five-year mortality by representation group. The overall model was adjusted for cancer stage, subtype, chemotherapy intensity, and year of EBC diagnosis. Stratified models were adjusted for cancer stage, individual treatment regimen (due to lack of chemotherapy intensity variation within subtype), and year of EBC diagnosis. Results: Of 11,770 patients, most were aged 45-69 (71%), White (72%), diagnosed with stage II (51%), or HR+HER2- EBC (56%). Unrepresented patients (7%) were categorized due to comorbidities (76%), concurrent cancer (22%), or both (2%). Underrepresented patients (45%) were categorized based on age (44%), race/ethnicity (39%), or both (17%). The remaining patients were well represented in trials (48%). In adjusted models, unrepresented patients had almost three times the hazard of death than well-represented patients (HR 2.71, 95% CI 2.08-3.52; Table). The hazard of death for underrepresented versus well-represented patients was similar (HR 1.19, 95% CI 0.98-1.45). Comparable results were seen in EBC subtype-specific models. Conclusions: Over half of patients in this study would be considered underrepresented or unrepresented in clinical trials due to age, comorbidity, or race/ethnicity. Patients considered unrepresented in trials experienced poorer survival compared to those well-represented. Trialists should ensure study participants reflect the real-world disease population to support evidence-based decision making for all individuals with cancer.