Intermittent Letrozole Therapy in Postmenopausal Women With Metastatic Breast Cancer

* This study involves treating participants with intermittent AI therapy. The AI will be stopped at the time they enter the study. We plan on monitoring a marker in the participants blood called CA 15-3 (or CA 27.29) every 4 weeks to help us make a decision of when to re-start treatment with letrozole (femara). This marker is known to rise when disease is progressing and drop when the disease is responding to treatment. We will be stopping and re-starting therapy based on the changes of CA 15-3 in the participants blood. * In addition to bloodwork, the following tests and procedures will be performed on a monthly basis: medical history; physical examination and performance status. * Every 8 weeks the following will be performed: Tumor assessment by physical exam (if possible); Chest x-ray or CT scan or chest; CT scans of abdomen and pelvis; and bone scan.