PURPOSE: The aims of this study were to evaluate the clinicopathologic characteristics and the prognosis of patients with estrogen receptor negative/progesterone receptor positive (ER-/PR+) breast cancer. METHODS: One thousand five hundred seventy patients were stratified according to ER/PR phenotype and our study focused on the ER-/PR+ phenotype. The clinicopathologic characteristics and the prognosis of patients with the ER-/PR+ phenotype were compared with those of patients with ER+ (ER+/PR- or ER+/PR+) breast cancer. RESULTS: The mean age at diagnosis was 47.1 years (range, 20-88) and the mean follow-up was 65.2 months. The horjmone receptor phenotype was ER-/PR+ in 75 cases (4.8%) and ER+ (ER+/PR+ or ER+/PR-) in 917 cases (58.4%). A patient age <50 (p=0.001), a high histologic grade (p=0.004) and C-erbB2 overexpression (p=0.006) were more frequent for the patients with the ER-/PR+ tumors. There was a significant difference between the two groups for the mean age (p<0.001). The 5 year and 10 year disease-free survival (DFS) rates of the ER-/PR+ group were 67.2% and 55.3%, respectively, and those of the ER+ group were 84.9% and 73.1%, respectively (p<0.001). The 5 year and 10 year overall survival (OS) of the ER-/PR+ group were 82.4% and 62.6%, respectively, and those of ER+ group were 93.4% and 83.3%, respectively (p=0.001). In the under 50 year old patients, the 5 year DFS and OS of the ER-/PR+ group were 67.5% and 85.8%, respectively, and those of ER+ group were 86.3% and 95.8%, respectively. There were significant differences between two groups for the DFS and OS (p<0.001). CONCLUSION: ER-/PR+ tumors have more aggressive clinicopathologic features than ER+ tumors. Furthermore, in the under 50 year old patients, ER-/PR+ tumors showed a worse prognosis than did the ER+ tumors. Consequently, treatment modality and the prognosis of the patients with ER-/PR+ tumors probably need to be altered from those of the patients with ER+ tumors.