Twenty-eight patients with a total of 35 suspect breast masses underwent positron emission tomography (PET) with [fluorine-18] 2-deoxy-2-fluoro-D-glucose (FDG) in order to study the utility of this technique in the evaluation of breast cancer. FDG PET allowed discrimination between eight benign and 27 malignant breast masses, with a sensitivity of 96% and specificity of 100%. Among the malignancies, there was a significant correlation between normalized FDG uptake and nuclear grade (P = .006). In addition, the results of PET imaging were compared with results of axillary node dissection in 20 cases of breast cancer. PET allowed correct categorization of 10 of 10 axillae as negative (specificity = 100%). PET results were equivocal in one axilla and positive in the remaining nine of 10 axillae with positive dissection results (sensitivity = 90%). The authors conclude that FDG PET may give useful information on breast masses and axillary node status prior to surgery.