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Background/Aims: Estrogen receptor (ER) is the prototype therapy predictive marker in oncology. The ER is now known to exist in two main forms with similar overall structure: ER-α and ER-β. Both forms may be expressed in breast cancer. The aim of this study was to examine breast cancer outcome in relation to expression of ER-β. Methods: In this investigation, we measured the expression of ER-α protein and ER-β mRNA in 121 extracts of invasive breast cancer. Association of expression with clinical outcome was examined using Kaplan-Meier and Cox regression analyses. Results: While ER-α expression was associated with good patient outcome [hazard ratio (HR) for death from breast cancer 0.37; 95% confidence interval (CI) 0.17–0.84; p = 0.017], ER-β predicted poor outcome (HR for death from breast cancer 2.49; 95% CI 1.10–5.63; p = 0.028). Conclusion: Based on these findings, we conclude that ER-β may have a different biological role from that of ER-α in breast cancer.