Arimidex (chemical name: anastrozole) is an aromatase inhibitor approved by the U.S. Food and Drug Administration (FDA) to treat:

• post-menopausal women diagnosed with hormone-receptor-positive, early-stage breast cancer after surgery (or possibly chemotherapy and radiation) to reduce the risk of the cancer coming back
• post-menopausal women diagnosed with advanced-stage or metastatic hormone-receptor-positive breast cancer 

Arimidex won't work on hormone-receptor-negative breast cancer....