For breast cancer patients with relapsed or recurrent disease, the presence of circulating tumor cells in the blood is predictive of poor survival. Circulating tumor cells can also be measured in the bone marrow, a method previously used to predict outcome of patients with early-stage breast cancer. The use of bone marrow fluids, however, has not been fully studied among women with more-advanced breast cancer. As well, using bone marrow fluid requires a more-invasive bone marrow aspiration to withdraw blood and fluid for examination.