Abstract Background: Black and younger women have poorer breast cancer outcomes. Using a diverse population-based study, we examined the role of biology as measured by genomic assays in outcome disparity among clinically HR+/HER2- women. Methods: Data and biospecimens from the Carolina Breast Cancer Study (CBCS, including 2,103 non-metastatic, invasive breast cancers) were used to perform RNA-based classification according to molecular subtype and research versions of known prognostic assays, the 21-gene Recurrence Score (RS) and the PAM50-based Risk of Recurrence (ROR-PT) score. Prevalence odds ratios (PORs) and 95% confidence intervals (CIs) for subtype by race and age were estimated. Results: Black women had higher frequency of Luminal B [POR (95% CI): 2.08 (1.60, 2.71)], HER2-enriched [POR (95% CI): 2.01 (1.45, 2.79)], and Basal-like tumors [POR (95% CI): 3.51 (2.81, 4.40)] compared to non-Black women. Similarly, younger (< 50 years) women had higher frequency of Luminal B [POR (95% CI): 1.57 (1.21, 2.04)], HER2-enriched [POR (95% CI): 2.03 (1.46, 2.82)], and Basal-like tumors [POR (95% CI): 2.40 (1.92, 3.01)]. Additionally, within clinically-defined HR+/HER2- tumors, Black women had higher frequency of high ROR-PT among younger women [POR (95% CI): 2.88 (1.19, 6.97)], but this association was attenuated among older women [POR (95% CI): 1.99 (0.84, 4.71)]. Race was not significantly associated with the 21-gene RS among younger or older women. Conclusion: While Black and younger women with clinically-defined HR+/HER2- often have higher burden of non-Luminal/high genomic risk tumors, PAM50 and 21-gene assays show different demographic patterns and heterogeneity within age- and race-defined groups, underscoring the value of genomic testing in understanding outcome disparities. Citation Format: Sanah Vohra, Sarah Van Alsten, Joannie M. Ivory, Alina Hamilton, Xiaohua Gao, Erin Kirk, Joseph Nsonwu-Farley, Ebonee Butler, Brianna Taffe, Charles M. Perou, Lisa Carey, Melissa Troester. PAM50 and 21-gene recurrence scores in younger and Black women with breast cancer [abstract]. In: Proceedings of the 2022 San Antonio Breast Cancer Symposium; 2022 Dec 6-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2023;83(5 Suppl):Abstract nr PD1-08.