The most common site of metastasis from breast cancer is bone. A major problem in treating bone metastases is the lack of objective measures of response to systemic chemotherapy or hormonal therapy. This occurs because abnormalities in bone scintigraphy and radiography, which are based on secondary manifestations of metastases (reactive new bone formation or bone destruction) are not present in all patients, when present indicate relatively advanced disease, and may or may not change when the cancer responds to or progresses during treatment. Thus patients with unresponsive cancers may suffer unnecessary toxicity from prolonged treatment; patients with responding cancers may have treatment discontinued prematurely; and many patients are ineligible for clinical trials of new treatments because they lack objective indicators of response. Our preliminary study in 33 patients shows that MRI directly visualizes metastatic deposits within bone marrow, does not rely on secondary manifestations, and reveals multiple small metastases not evident in radiography or scintigraphy. We propose to test the hypothesis that MRI is a reliable, objective means to assess response of bone and bone marrow metastases of breast cancer to treatment with systemic therapy. Our approach is to conduct a prospective trial in which concordance with other indices of response is examined and clinical endpoints (duration of complete response, time to progression, and survival) are used to determine if MRI response categories are significant predictors of prognosis.