This is a prospective study evaluating the role of Positron Emission Tomography (PET scan) in breast cancer. The radiopharmaceutical [18F] 2-deoxyglucose will be used as both an imaging modality and to evaluate tumor metabolism in patients with breast cancer. Patients with stage II, stage IIIA or IIIB, or stage IV breast cancer and evaluable disease in the breast and/or at metastatic sites will be studied. Patients will be injected intravenously with [18F]2-deoxyglucose and a PET scan conducted over 90 minutes. Examination of the role of PET scan in assessing the response of breast cancer to chemotherapy, especially in stage II and stage III patients, will be done. Up to three PET scans within one year may be performed. Findings by PET scan will be correlated both with those of concurrent imaging techniques (x-ray, CT scan, MRI, bone scan or mammogram), and with histologic, biochemical, and flow cytometric information on the tumor, and with findings in subsequent surgical specimens. Changes in tumor glucose metabolism with treatment will be assessed by PET imaging studies and by biochemical methods when possible.