The presence of CTC in the peripheral blood, as detected by the CellSearch® Circulating Tumor Cell Kit, is associated with decreased progression free survival and decreased overall survival in patients treated for metastatic breast cancer. This test is to be used as an aid in the monitoring of patients with metastatic breast cancer. Serial testing for CTC should be used in conjunction with other clinical methods for monitoring metastatic breast cancer. Evaluation of CTC at any time during the course of disease allows assessment of patient prognosis and is predictive of progression free survival and overall survival.