Background: Persons experiencing homelessness (PEH) suffer from poor health outcomes, including worse cancer mortality, compared to persons not experiencing homelessness. A portion of the disparity in cancer outcomes is attributable to reduced access to cancer screening, leading to more advanced-stage disease and a higher risk of death compared to the general population. Data regarding cancer screening rates in PEH are scarce. We therefore sought to evaluate baseline rates of breast cancer screening in PEH. Methods: All patients presenting for care from January 1, 2014 onward at a hospital system spanning five counties in a populous Midwestern state were screened for homelessness. Homelessness was identified by two criteria: presence of the Z-code for homelessness (Z59) in the patient’s electronic medical record, and/or patient’s address on record listed as an address matching that of a regional homeless shelter, transitional housing, or “homeless.” Identified PEH were maintained in a prospective registry. For each female PEH in the screening age range, billing data for completed breast cancer screening mammography performed in the previous five years (1/1/17-12/31/21) were extracted (CPT codes 77063, 77067). Data were also extracted for a cohort of non-PEH patients eligible for screening. Demographic and clinical data were extracted for all patients. This study was approved by the hospital system’s IRB. Results: A total of 3,474 female (biological sex) PEH were identified, with 1,320 eligible for screening mammography (alive and between the ages of 40 and 79) in the study timeframe. The median age was 53.5 years old; 44% were Black, 48% White, 8.5% unknown/other race, and 3% Hispanic ethnicity. 28% of PEH were uninsured, and 67% had government insurance; 66% had an assigned primary care physician (PCP). Of PEH eligible for screening mammography, 237 (18%) had at least one screening mammogram during this five-year interval (2017, 2.2%; 2018, 4.3%; 2019, 3.6%; 2020, 3.7%; 2021, 4.3%). In a cohort of 6,240 non-PEH eligible for screening over the same timeframe, the screening mammography rate was 32%, which was significantly higher than the screening rate for PEH (p < 0.00001). Compared to PEH who did not undergo screening mammography, PEH who underwent screening mammography were more likely to have an assigned PCP (90% vs 38%, p < 0.00001), to be a non-current tobacco user (56% vs 35%, p < 0.00001), and to be a non-current illicit drug user (84 % vs 68%, p = 0.0015). PEH who underwent screening mammography were significantly less likely to be uninsured (12% vs 31%, p < 0.00001). Conclusions: In the largest study of its kind to date, we identified low rates of breast cancer screening in female PEH. Interventions to increase breast cancer screening in this vulnerable population are urgently needed and may include increased access to PCPs, tobacco and drug cessation programs, and provision of health insurance.