ABSTRACT This application is being submitted in response to the NOSI identified as "NOT-CA-24-032". Asian American (AA) is the fastest-growing racial/ethnic group within the USC Norris Comprehensive Cancer Center (NCCC) catchment area. Breast cancer is the most frequently diagnosed cancer among AA women, contributing to 10- 20% of cancer-related deaths. In Los Angeles County (LAC), the breast cancer incidence in AA women has seen a steady increase over time, and the annual percentage increase for breast cancer among AA women was the highest of any reported ethnic groups. Data are limited, but variations in tumor characteristics, screening, and care delivery/patterns of care and outcomes across disaggregated AA ethnic groups have been reported. Health behaviors, cultural factors, and social barriers, alongside healthcare and structural factors, can contribute to these differences. Despite the significance of these multilevel factors, their impact on breast cancer care delivery and outcomes in AA women, particularly within specific AA groups, remains understudied. Our objective is to address the gap of knowledge of breast cancer among AA women and examine breast cancer characteristics and disparities in care patterns and outcomes within heterogenous AA groups. We hypothesize that tumor characteristics, health behaviors, cultural, and structural factors vary within AA groups, leading to heterogeneous breast cancer outcomes. In this supplement, we plan to recruit AA breast cancer cases within the NCCC catchment area by leveraging the existing infrastructure of an ongoing breast cancer study, focusing on Chinese, Korean, and Filipino women, the largest Asian ethnic groups in LAC. We will collect comprehensive data on tumor characteristics, health behaviors, cultural, and structural factors, using quantitative and qualitative methods, to evaluate how within group diversity among AA influences breast cancer outcomes. The specific aims are to 1) examine breast tumor characteristics and prior screening patterns by behavioral, social and structural factors in recently diagnosed breast cancer cases across 3 ethnic groups (Chinese, Filipino and Korean) in our catchment area through the expansion of an ongoing Norris breast cancer cohort; 2) examine patterns of care and receipt of/timeliness to screening follow-up, diagnosis, and treatment and factors that influence breast cancer care outcomes; 3) inform the development of a multilevel intervention, that builds upon existing infrastructure from the USC Norris Ralph Lauren Center, to address breast cancer care and navigation needs specific for AAs through stakeholder inputs. Understanding the unique multilevel factors that contribute to barriers to screening and optimal care across disaggregated AA groups will inform targeted interventions critical to improving breast cancer outcomes within this population, ultimately reducing disparities among its heterogeneous segments. Findings will inform the development o...