Positron emission tomography (PET) has been utilized to image several types of tumors in vivo. PET has the advantage of providing functional information not obtainable with anatomic imaging modalities, such as mammography, ultrasound and magnetic resonance imaging. When used in conjunction with the glucose analog [fluorine-18] 2-deoxy-2-fluoro-D-glucose (FDG), PET has shown a high correlation with histological grade in tumors of the brain and musculoskeletal system. FDG uptake by astrocytomas correlates inversely with patient survival. Recently, PET-FDG has successfully imaged both primary breast carcinomas and axillary node metastases. Preliminary data demonstrate a significant difference in FDG uptake between benign and malignant breast lesions. There is a moderate correlation between FDG uptake in the primary malignancies and the presence of lymph nodes as detected by axillary node dissection. As treatment for breast cancer is determined by individual relapse risk, there is a need for more accurate stratification of patients into risk categories. Currently, the presence or absence of axillary nodal disease is the single most important prognostic factor. To make this determination from axillary node dissection has attendant costs and morbidity. This proposal seeks funding to study 100 women who have solid breast masses with PET-FDG prior to surgery. This study will assess the ability of PET-FDG to discriminate between benign and malignant breast lesions. The accuracy of PET-FDG at predicting the presence or absence of axillary node metastases as determined by axillary node dissection will be assessed. Finally, the uptake of FDG will be compared with patient survival statistics to determine if FDG uptake may be a strong prognostic factor for patient survival. If PET-FDG demonstrates new prognostic information for breast cancer, it will have profound implications on patient management.