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Objective:Breast carcinomas positive for the estrogen receptor (ER+) but negative for the progesterone receptor (PR−) have unfavorable prognostic features and are resistant to tamoxifen therapy. The goal of this study was to highlight the significance of PR− breast carcinomas.Methods:Therefore, 146 breast carcinomas comprising 87 ER+/ PR+ and 59 ER+/PR− carcinomas were examined. These two groups were compared in terms of age; tumor type; tumor size; histologic grade; presence of an in situ component; lymphovascular and perineural invasion; and ER, PR, c-Erb B2, Ki-67, and epidermal growth factor receptor (EGFR) status.Results:While the number of metastatic lymph node and related pN2+pN3 tumors were found to be significantly higher in the ER+/PR− group, the differences with respect to the tumor size, metastatic lymph node size, and frequency of lymphovascular invasion were nearly significant.Conclusion:ER+/PR− tumors have an unfavorable prognosis and show a clinical behavior closer to triple negative ones, although classified as luminal tumors. Revealing the mechanisms causing these differences will enhance the success of breast cancer therapy.