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Background: Asian women with breast cancer are often studied in aggregate, belying significant intragroup diversity. The authors sought to examine differences in breast cancer characteristics and outcomes among Asian women., Methods: Asian, non-Hispanic Black, Hispanic, and non-Hispanic White women aged 18 years and older who were diagnosed with breast cancer from 1990 to 2016 were identified in the Surveillance, Epidemiology, and End Results 18 database. Asian patients were subclassified as Chinese, Japanese, Korean, Filipino, Vietnamese, South Asian (Asian Indian or Pakistani), Southeast Asian (SEA, i.e., Cambodian, Laotian, Hmong, or Thai), or other Asian. Unadjusted overall survival (OS) and cancer-specific survival (CSS) were estimated using the Kaplan-Meier method. Cox proportional hazards models were used to estimate adjusted OS and CSS., Results: In total, 910,415 women were included (Asian, n = 63,405; Black, n = 92,226; Hispanic, n = 84,451; White, n = 670,333). Asian women had higher rates of human epidermal growth factor receptor 2 (HER2)-positive disease compared with White women (18.7% vs 13.8%) and had the highest 10-year unadjusted OS and CSS among all racial/ethnic groups (all P < .001). SEA women had the highest rates of stage IV disease at presentation, whereas Japanese women had the lowest rates (5.9% vs 2.7%; P < .001). Japanese women had the highest 10-year unadjusted CSS (89.4%; 95% confidence interval, 88.7%-90.1%) of any distinct Asian group, whereas SEA women had the worst unadjusted CSS (78%; 95% confidence interval, 74.1%-81.3%; P < .001). After adjustment, SEA women had the worst OS of any Asian group and were the only Asian group without improved OS compared with White women (reference category; P = .08)., Conclusions: Breast cancer characteristics and outcomes vary significantly among Asian women. Future research should consider disaggregation by country or region of origin to identify subgroups that are at risk for worse outcomes than aggregated data may suggest., Lay Summary: Asian women with breast cancer are frequently studied as a single entity. However, Asian ethnic groups differ greatly by country of origin, genetic ancestry, disease frequency, socioeconomic status, patterns of immigration, as well as dietary and cultural practices. Women of different Asian ethnicities vary significantly with regard to cancer characteristics, such as mortality and tumor subtype. Future research should disaggregate these populations to better understand, treat, and counsel Asian patients with breast cancer., (© 2021 American Cancer Society.)