Abstract Background: The impact of financial toxicity (FT) on cancer patients is significant in terms of lost productivity and poor quality of life (QoL). Past studies have reported negative influence of high FT on patients actively undergoing treatment for breast cancer. However, existing data largely comes from quaternary referral centers, and the populations surveyed may not be generalizable in terms of income and insurance status across the United States. There is also limited understanding regarding the relationship between FT measured throughout the continuum of cancer care, demographics, disease factors, and QoL indicators in breast cancer survivors, particularly those from underserved communities. Methods: Breast cancer survivors who received treatment between 2015 and 2019 at Loma Linda University Cancer Center located in San Bernardino County, CA were invited to complete an anonymous online survey assessing demographics, disease history, FT, and QoL. A modified version of the Comprehensive Score for Financial Toxicity (COST) survey was used to assess FT at baseline and after treatment. The patient-reported outcomes measurement information system (PROMIS) survey was used to assess QoL. Demographic data were summarized using descriptive statistics. Associations between disease factors and FT measured at baseline and after treatment were analyzed using multivariable linear regression. Positive COST score coefficients indicate lower FT, while negative COST score coefficients indicate higher FT. Correlations between FT and QoL were evaluated using the Pearson correlation. Results: A total of 407 surveys were sent to breast cancer survivors who met study criteria, of whom 16% responded. Amongst 65 patients included in this analysis, the median age was 64 (IQR 56, 70), 68% were white (n = 44), 18% were unemployed, 13% had a high school or lower level of education, and 16% had an annual income < $30,000. Mean COST score at baseline and after treatment were 21.6 ± 9.6 and 12.9 ± 7.8, respectively. During treatment, 21% of patients turned down or skipped treatment, 23% quit their job, and 15% reported family members quit their job. At baseline, lower FT was significantly associated with age > 80 (6.2 95% CI: 4.1 to 8.3), presence of a college (4.5 95% CI: 1.4 to 10.4) or graduate degree (5.2 95% CI: 4.7 to 6.2), current employment (2.7 95% CI: 2.0 to 3.4), retiree status (6.7 95% CI: 0.4 to 13.0), and household income > $30,000 (5.9 95% CI: 2.7 to 9.1) (p < 0.05), while higher FT was significantly associated with Hispanic ethnicity (-4.1 95% CI: -6.7 to -1.5), Medicare (-1.6 95% CI: -5.2 to -2.0), and Medicaid insurance (-6.9 95% CI: -12.5 to -1.2) (p < 0.05). After treatment, higher FT was significantly associated with receiving 3 or more combined modality treatments (-5.6 95% CI: -9.7 to -1.5) and having 5 or more treatment-related side effects (-6.0 95% CI: -10.4 to -1.6) (p < 0.05). Finally, FT after treatment overall correlated positively with physical and mental health (Pearson coefficient 0.63 and 0.60 respectively, p < 0.01). Conclusions: These findings suggest a significant association between FT, baseline demographics, treatment modalities, side effects, and quality of life in an underserved population of breast cancer survivors. In the future, FT should be assessed at baseline and throughout the continuum of breast cancer care to provide individualized assistance to patients facing financial strain, as it affects compliance to treatment which in turn can adversely affect cancer-related outcomes and QoL. Additional policies are needed to address the increasing cost of breast cancer care. Citation Format: Akhil Mehta, Jukes Namm, Ellen D'Errico, Eric Lau, Sharon Lum, Gayathri Nagaraj. Burden of financial toxicity in an underserved population of breast cancer survivors [abstract]. In: Proceedings of the 2021 San Antonio Breast Cancer Symposium; 2021 Dec 7-10; San Antonio, TX. Philadelphia (PA): AACR; Cancer Res 2022;82(4 Suppl):Abstract nr P4-11-25.