Objectives: Under-served groups are populations unrepresented or disengaged from medical research or services despite a disproportionately high healthcare burden. Under-served groups may be directly (age, pregnancy as examples) or indirectly excluded (provision of written information in one language only as an example) from trial enrollment by strict eligibility exclusions. The purpose of our study was to assess eligibility criteria in published phase III breast cancer clinical trials to determine whether they excluded underserved groups either directly or indirectly. Study Design and Setting: Medline was searched for phase III randomized controlled trials evaluating interventional drugs for breast cancer in high-impact journals published between January 1st, 2010 and December 31st, 2020. A total of 5133 eligible trials were returned, and 40 were selected, by simple randomization, for inclusion. Results: All 40 trials had multiple exclusions that affected the recruitment of underserved groups. Clinical or scientific rationale for the recorded inclusion and exclusion criteria was underreported in 39 of 40 trials. Conclusion: Clinical trial eligibility criteria exclude underserved groups from breast cancer trials. Trialists should provide a justification for each eligibility criterion, and funders, reviewers, ethics committees, and others should demand one. Without this, underserved groups will remain just that: underserved. (C) 2022 The Author(s). Published by Elsevier Inc.