BACKGROUND: The aim of this study was to evaluate the prognostic impact of quantitative estrogen receptor (ER) expression at relapse for ER-positive breast cancer with ER-positive recurrence. PATIENTS AND METHODS: A total of 81 patients with ER-positive primary breast cancer and ER-positive paired recurrence were included. ER expression was evaluated as the percentage of tumor cells staining for ER under immunohistochemistry. Samples were defined as ER-high (ER>50%) or ER-low (ER≥10% and ≤50%). RESULTS: Quantitative ER expression on relapse biopsy was an independent prognostic factor for overall survival in multivariate analysis, both as a continuous (hazard ratio=0.8; 95% confidence interval=0.7-0.92, p=0.001) and as a categorical (ER-high vs. ER-low; hazard ratio=0.26; 95% confidence interval=0.11-0.59, p=0.001) variable. Patients whose status changed from ER-high (primary BC) to ER-low (relapse) had the poorest outcome, with a 10-year overall survival rate of 14%. CONCLUSION: Even in the case of maintenance of ER-positivity on primary and relapse of breast cancer, recurrence biopsy provides prognostic information.