Breast cancer is the most common cancer in Taiwanese women. Most of them were diagnosed in relatively early stages. Only 700 metastatic breast cancer were registered in Official Cancer Registration 2012, while more than11,305 were registered as early breast cancer. With broadly application of aromatase inhibitors, hormonal-receptor-positive breast cancer patients can survive longer and longer. However, there is no prevalence of metastatic breast cancer can be available yet. NHIA database coverage is more than 90% population in Taiwan, thus it can be an appropriate surrogate of prevalence. In this retrospective database analysis, we will explore the real world experience on aromatase inhibitor use for breast cancer in Taiwan.