Background: Estrogen receptor (ER) positive breast cancer (BC) is a heterogeneous disease, with ongoing debate on the optimal cut off point for clinically relevant ER expression. Tumors harboring ≤10% ER expression are associated with poor outcomes. We used a real-world database to assess prognostic and predictive value of an alternative ER expression cut points. Methods: This retrospective cohort study used the nationwide Flatiron Health electronic health record (EHR)-derived de-identified database. During the study period, the de-identified data originated from approximately 280 US cancer clinics (̃800 sites of care). We evaluated the association between ER expression (assessed locally by immunohistochemistry according to ASCO/CAP guidelines) with tumor characteristics, and treatment patterns of patients with early-stage BC (stage I-III) using descriptive statistics. Recurrence free survival and overall survival was defined as time in months from date of surgery until the data of documented cancer recurrence or death respectively. We used Kaplan Myer survival curves to calculate recurrence free (RFS) and overall survival (OS) of patients with ER low, ER intermediate and ER high tumors. To define an alternative ER expression cut point, the data set was divided into 2/3 training and 1/3 test data. A cut point analysis was performed on the training data set to find the optimum cut point of ER+ staining based on correlation with recurrence free survival as the outcome. Results: Among 4,697 ER positive early-stage BC patients, 83 (1.8%) had ER low (ER expression :1-10%) and 36 (0.8%) had ER intermediate BC (11-20%). Median follow up time was 63 months (range 24-84). ER low tumors were associated with higher tumor grade, larger size, and higher axillary tumor burden compared to ER high positive tumors ( > 20% ER expression). African American patients had a higher prevalence of both triple negative and ER low positive BCs compared to ER high tumors- 21%, 22%, and 8% respectively. No significant differences in patient- or tumor-associated characteristics were observed between Low ER and intermediate-ER positive BC patients. Both ER low and intermediate positive tumor patients had survival outcomes similar to patients with TNBC and worse than ER high positive tumors (p < 0.001). No significant correlations between endocrine therapy and RFS or OS were observed among patients with either ER low or intermediate BCs (HR 1.47 and 2.57, p > 0.05; respectively). Sensitivity analysis showed that tumors with ≤ 20% ER expression were associated with worse RFS in both univariate and multivariate analyses (p < 0.05). Conclusions: These findings suggest that patients with ER expression rates ≤ 20% have poor outcomes and derive minimal benefit from endocrine therapies.