Background: Breast cancer survivors on aromatase inhibitors (AIs) can develop endothelial dysfunction, a measure of increased CV risk. Stress modifying techniques such as MBSR may improve CV function and quality of life (QOL). We present data of a pilot study assessing the impact of MBSR on endothelial dysfunction and psychosocial outcomes. Methods: 31 women with locally advanced breast cancer and taking an AI were randomized 1:1 to 8 weekly MBSR classes or wait-list control. Subjects with tobacco use, known hypertension or hyperlipidemia were excluded. Participants completed surveys and vascular testing at baseline and 10 weeks. With the COVID-19 pandemic, MBSR classes were held on a virtual platform. Endothelial function was measured using the Endo-PAT2000 system; reductions in EndoPAT ratio (<1.6) are indicative of worsening endothelial function and increased CV risk. Questionnaires included the CDC’s 4-item Healthy Days Core Module (HRQOL), Patient Health Questionnaire (PHQ)-8 for depression, the Perceived Stress Scale, and the Generalized Anxiety Disorder Scale (GAD-7). Functional test markers were compared between groups using t-tests. Results: Baseline mean age was 59 years, body mass index was 28.4 kg/m², mean systolic blood pressure was 128.1 mmHg and cholesterol was 226.9 mg/dL. These measures did not differ between groups. Baseline EndoPAT ratio (0.60) and follow up (0.52) were low in all participants. As statewide COVID-19 cases spiked, 3/31 participants did not return for a second assessment, and for another 5 participants, one EndoPAT ratio measurement was not interpretable. Both groups showed a decline in EndoPat ratio pre-to-post, but the decline among MBSR participants (all of whom completed the intervention) was five times less than that observed among controls [Table]. Groups did not differ on QOL or psychosocial outcomes. Conclusions: Breast cancer survivors on AIs have endothelial dysfunction, a measure of increased CV risk. Stress reduction techniques, using virtual platforms, are feasible and acceptable to this patient population, and may help improve endothelial function. Further research and longer follow up is warranted in understanding the long-term impact of these interventions on CV risk and psychosocial well-being.