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IMPORTANCE: Patients who are uninsured and belong to racial and ethnic minority groups or have low socioeconomic status have suboptimal access to health care, likely affecting outcomes. The association of the Affordable Care Act’s Medicaid expansion with survival among patients with metastatic breast cancer is unknown. OBJECTIVE: To examine the association between Medicaid expansion and mortality disparity among patients with de novo stage IV breast cancer. DESIGN, SETTING, AND PARTICIPANTS: Cross-sectional, population-based study of survival using Cox proportional hazards regression and difference-in-difference (DID) analysis of data from the National Cancer Database and patients diagnosed as having de novo stage IV breast cancer between January 1, 2010, and December 31, 2016, residing in states that underwent Medicaid expansion on January 1, 2014. The preexpansion period was January 1, 2010, to December 31, 2013; the postexpansion period was January 1, 2014, to December 31, 2016. Data were analyzed between September 4, 2020, and November 16, 2021. EXPOSURES: Comparison of survival improvement between patients of racial and ethnic minority groups and White patients in the preexpansion and postexpansion periods. Because of small numbers in the specific racial and ethnic minority groups, these patients were combined into 1 category for comparisons. MAIN OUTCOMES AND MEASURES: Overall survival (OS) and 2-year mortality rate. RESULTS: Among 9322 patients included (mean [SD] age, 55 [7] years), 5077 were diagnosed in the preexpansion and 4245 in the postexpansion period. The racial and ethnic minority group comprised 2545 (27.3%), which included 500 (5.4%) Hispanic (any race), 1515 (16.3%) non-Hispanic Black, and 530 (5.7%) non-Hispanic other including 25 (0.3%) American Indian or Alaska Native, 357 (3.8%) Asian or Pacific Islander, and 148 (1.6%) unknown, and 6777 (72.7%) were in the White patient group. In the preexpansion period, White patients had increased OS compared with patients of racial and ethnic minority groups (adjusted hazard ratio [aHR], 1.22; 95% CI, 1.10-1.35); this difference was not observed in the postexpansion period (aHR, 0.96; 95% CI, 0.86-1.08). A reduction in 2-year mortality was observed between the preexpansion and postexpansion periods (32.2% vs 26.0%). The adjusted 2-year mortality decreased from 40.6% to 36.3% among White patients and from 45.6% to 35.8% among patients of racial and ethnic minority groups (adjusted DID, −5.5%; 95% CI, −9.5 to −1.6; P = .006). Among patients in the lowest income quartile (n = 1510), patients of racial and ethnic minority groups had an increased risk of death in the preexpansion period (aHR, 1.28; 95% CI, 1.01-1.61) but lower risk in the postexpansion period (aHR, 0.75; 95% CI, 0.59-0.95). In this subset of patients, those of racial and ethnic minority groups had a greater reduction in 2-year mortality compared with White patients (adjusted DID, −12.8%; 95% CI, −22.2 to −3.5; P = .007). CONCLUSIONS AND RELEVANCE: In this cross-sectional study, survival differences observed between patients of racial and ethnic minority groups and White patients in the preexpansion period were no longer present in the postexpansion period. A greater reduction in 2-year mortality was observed among patients of racial and ethnic minority groups compared with White patients. These results suggest that policies aimed at improving equity and increasing access to health care may reduce racial and ethnic disparities in breast cancer outcomes.