139 Background: Food insecurity, housing instability, and transportation challenges are modifiable health-related social needs (HRSN) that have been associated with worse health outcomes in patients with cancer. Nonadherence to medications is also associated with decreased survival and other poor outcomes. Less is known about the association between HRSN and medication adherence in this patient population. Methods: We implemented screening for HRSN among breast oncology patients at our urban academic medical center, primarily through the electronic patient portal. Patients were offered an 8-item HRSN survey with questions covering food insecurity, housing, transportation needs, and emergency department visits. Any level of food hardship, housing instability, or transportation challenges reported during the study period (11/2/22-4/20/24) constituted a positive screen for that HRSN. We also screened patients for nonadherence to medications (any medication) using the 3-item DOSE-Nonadherence questionnaire. Pearson's Chi-squared tests and univariate logistic regressions were used to compare differences between subsets. Results: During the 18-month study period, 2945 patients completed at least 1 item on the HRSN questionnaire, of which 765 (26.0%) screened positive for at least 1 HRSN. Among patients who responded to each HRSN domain, 555/2820 (19.7%) screened positive for food insecurity, 374/2650 (14.1%) for housing instability, and 255/2656 (9.6%) for transportation needs. Food-insecure patients also reported more housing (43.1% v 5.6%, p<0.001) and transportation needs (27.7% v 4.0%, p<0.001) compared to food-secure patients. When comparing the cohort of patients who screened positive for at least 1 HRSN vs those who screened negative, there was a higher proportion of Hispanics (55.3% vs 25.1%, p<0.001), non-Hispanic Blacks (15.3% v 10.0%, p<0.001), and Spanish speakers (41.2% v 17.2%, p<0.001). Patients with any HRSN were more likely to self-report medication nonadherence compared to those with a negative HRSN screen (44.2% v 35.3%, OR 1.45, 95% CI 1.18-1.79). When looking at each individual HRSN domain, food-insecure patients were more likely to self-report medication nonadherence than food-secure patients (45.9% v 35.9%, OR 1.51, CI 1.19-1.92); this was also true for patients with housing instability (45.7% v 36.4%, OR 1.47, CI 1.12-1.92) and transportation needs (49.1% v 36.5%, OR 1.68, CI 1.23-2.31). Conclusions: More than a quarter of breast cancer patients self-reported at least 1 HRSN, with food insecurity being most frequently reported. Patients with at least 1 HRSN had 45% increased odds of medication nonadherence. Further research is warranted to understand the financial, social, psychological, and healthcare-related factors that may underlie this association, and to design interventions targeting at-risk cancer populations.