OBJECTIVE Estrogen receptor (ER) and progesterone receptor (PR) have been used as indicators of endocrine system status since the mid-1970s in the clinical management of breast cancer. The predictive role of ER in endocrine therapy is undisputed, but the prognostic value of PR is still debated. The aim of this study was to investigate the clinical characteristics and prognosis of ER positive breast cancer with different PR expression levels. METHODS A population cohort of 3,030 primary invasive ER positive breast cancer patients from a single cancer center underwent surgery and received adjuvant endocrine therapy from 2004 to 2010. The clinical and biological features of these patients with high PR-expressing tumors were compared with those of patients with low PR-expressing tumors. The follow-up data for disease-free survival (DFS), overall survival (OS), and breast cancer specific survival (BCSS) was obtained from 2,778 patients. Cox regression analysis was used to correlate biomarkers and tumor characteristics with DFS, OS, and BCSS. RESULTS Tumors with low PR expression had more invasive pathological features and biological indexes than those with high PR expression. Low PR expression was an independent poor prognostic factor for DFS (p=0.014; hazard ratio [HR], 0.781; 95% confidence interval [CI], 0.641-0.950), OS (p=0.002; HR, 0.699; 95% CI, 0.560-0.873), and BCSS (p=0.005; HR, 0.714; 95% CI, 0.566-0.902). Furthermore, in low PR expressing tumors, patients who received chemotherapy had better DFS (p=0.002; HR, 0.449; 95% CI, 0.268-0.751), OS (p<0.001; HR, 0.341; 95% CI, 0.192-0.606), and BCSS (p<0.001; HR, 0.292; 95% CI, 0.156-0.549) than patients who did not received chemotherapy. CONCLUSIONS Patients with ER positive invasive breast cancer with low PR expressing tumors have a worse prognosis than those with high PR expressing tumors, and these patients can benefit from chemotherapy.