Background and purpose: The third generation of aromatase inhibitors (AI) in postmenopausal hormone receptor-positive patients is the routine treatments in endocrine therapy. The 500 mg fulvestrant showed clinical benefits in patients with previous AI treatment. This study aimed to access the efficacy and safety of 500 mg fulvestrant in estrogen receptor (ER) positive postmenopausal patients who had previous AI treatments with locally advanced and metastatic breast cancer. Methods: This study retrospectively analyzed the clinical data from 188 post-AI ER positive and (or) progesterone receptor (PR)-positive locally advanced and metastatic breast cancer patients treated with 500 mg fulvestrant in Fudan University Shanghai Cancer Center from Jul. 2011 to Dec. 2015. Primary end point was progression-free survival (PFS). Secondary end points were objective response rate (ORR)

clinical benefit rate (CBR) and safety profile. Results: After the median follow-up of 11.3 months

median PFS was 5.9 months (95%CI: 4.2-7.5)

CBR was 40.0% and ORR was 3.4%. COX proportional hazards regression analysis indicated that PFS was correlated with the number of metastatic sites (HR=1.92

95% CI: 1.2-2.9

P =0.002) and previous lines of chemotherapy (HR=1.52

95%CI:1.0-2.1

P=0.022). Six patients stopped the treatment for intolerable adverse events. Conclusion: The treatment of 500 mg fulvestrant has a favorable efficacy and safety in treatment of post-AI ER positive postmenopausal patients with metastatic breast cancer.