Positron Emission Tomography (PET) is a new imaging method employing radionuclide and tomography technique. In breast cancer PET has high sensitivity in detecting primary tumor and axillary node metastasis. From 1995 June to 1996 November, 27 patients had undergone breast operations following PET under impression of breast cancer in Seoul National University Hospital (SNUH). Whole body PET images were obtained beginning 60 minutes after infection of 370 MBq (10mCi) F-18 FDG (fluorodeoxy glucose). Regional scans were also obtained with transmission images. We compared PET results with those of physical examination and mammography. All cases were histologically confirmed. For primary tumor mass, diagnostic accuracy of PET was excellent (97%) compared with the physical examination (78%) and mammography (67%), For axillary lymph node metastasis, PET had an outstanding detection accuracy (96%), compared with the physical examination and mammography (74%, 60%, respectively). And whole body PET scan made it possible to see the all metastatic lesions at a glance in cases of metastatic or recurred breast cancer. There was likely correlation between Standard Uptake Value (SUV) and the number of axillary lymph node metastasis, but in this study, statistical significance was not proven because of small number of cases. PET also could detect breast cancer in paraffin augmented breast. We concluded that PET is very sensitive and accurate diagnostic tool for breast cancer and SUV, after more studies, could be used as an important prognostic factor.