Background: The Breast Cancer Index (BCI) has been validated to quantify an individualized risk of late recurrence and to predict likelihood of benefit from extended endocrine therapy in ER+ early stage breast cancer.The purpose of this study was to prospectively assess the impact of BCI i) to change treatment recommendations regarding extended endocrine therapy, and ii) to examine its effects on patient anxiety and decision conflict. Methods: Patients with stage I-III, ER+ breast cancer treated at the Yale Cancer Center who completed at least 3.5 yrs of adjuvant endocrine therapy were prospectively enrolled over 6 months in 2014. BCI was performed on FFPE samples from the original biopsy (bioTheranostics Inc.). Patients and physicians completed pre- and post-test questionnaires. Patients completed the Traditional Decisional Conflict Scale (DCS) and State Trait Anxiety Inventory Form Y (STAI) pre- and post-test. Results: 100 patients[mean age 61y (45-88), 80% postmenopausal, 57% stage I] were included in this study. Integration of BCI resulted in a change in treatment recommendation for 27% of patients. A majority of changes were based on identification of patients with low risk of late recurrence and low likelihood to benefit from extended endocrine therapy. Extended therapy was recommended for 75% patients pre- and for 55% post-testing. No extended therapy was recommended for 25% patients pre-and for 45% post-testing. Satisfaction increased in 38% of patients. After receiving results, 52% of patients changed their treatment decision. Patients experienced less anxiety (53%) and decisional conflict (50%) after receiving results. The STAI (p = 0.03) and DCS (p < 0.001) scores decreased significantly post testing (mean difference of -2 and -9, respectively) compared to baseline scores (31 and 20, respectively). Conclusions: The Breast Cancer Index led to changes in treatment recommendations regarding extended adjuvant endocrine therapy in 27% of cases and resulted in significantly less decision conflict and anxiety for patients. Overall, knowledge of BCI resulted in fewer recommendations for extended therapy and improved patient satisfaction. Clinical trial information: NCT02057029