PROACT - Pre-Operative Arimidex Compared To Tamoxifen

Inclusion Criteria: * Operable or potentially operable breast cancer * Invasive breast cancer proven to be hormone-sensitive (ER+/PR+) * Patients who might benefit from endocrine treatment prior to surgery * Postmenopausal * Signed written informed consent Exclusion Criteria: * Inoperable breast cancer * Patient unwilling to undergo surgery * Any reason why the patient may not be able to conform to study requirements * Any previous treatment for breast cancer * Previous Tamoxifen use as a part of breast cancer prevention trials * Not willing to stop taking drugs that affect sex-hormones such as HRT * Previous history of breast cancer * Previous history of invasive malignancy within the last 10 years * Any other severe disease that may prevent surgery, place the patient at risk, or influence the study results * Treatment with an experimental drug within the last 3 months * Risk of transmitting HIV, Hepatitis B or C