Breast cancer is the most common cancer and the second leading cause of cancer-related deaths among Canadian women. This disease can also occur in men, but is relatively rare. The use of aromatase inhibitors to prevent the recurrence of disease has become a clinically important therapy for the treatment of breast cancer. These drugs are effective and generally well tolerated in most patients, but some experience unwanted effects, such as thinning of the bones or osteoporosis. Other patients have disease recurrence indicating that there are different degrees of effectiveness of these medications. Currently it is not known which patients will respond to aromatase inhibitor therapy and which will be at risk for unwanted side effects, but is thought to be partly due to genetic differences in the genes responsible for the breakdown of drugs. Depending on a person's genes, the drug may be broken down, or metabolized, in different ways within their body. We will analyze blood samples to measure drug levels and learn how genetic differences in breast cancer patients affect the breakdown of aromatase inhibitors. Once we understand which genes contribute to unwanted effects, we can likely prevent these effects, which could lead to personalized therapy for breast cancer patients. This strategy will both enhance the treatment of breast cancer and improve the long term quality of life of patients.