The long-term objective of this proposal is to develop a method of treating lymph nodes in breast cancer patients that is substantially less morbid than current methods, yet still provides the same diagnostic and therapeutic benefits. An additional objective is to determine whether the presence of small "occult" metastases adversely impact overall and disease free-survival. Successful completion of this protocol will result in the demonstration of a less morbid method of managing lymph nodes in breast cancer patients and will also result in the education of hundreds of surgeons, nuclear medicine physicians, and pathologists in a standardized method of sentinel node management. In addition, this trial will result in the accrual and analysis of the world's largest series of occult node metastases, which will allow determination of the survival outcome of immunohistochemical positive- and hematoxylin-negative nodes. Due to the complexity of the sentinel node procedure, a careful training and monitoring program has been employed. Following successful training of a standardized protocol , surgeons randomize clinically node- negative breast cancer patients into Group 1 (axillary dissection and sentinel node resection) and Group 2 (sentinel node only and axillary dissection only, if the sentinel nodes are pathologically positive). The specific aims are to 1) determine whether sentinel node resection alone compared to axillary dissection plus sentinel node resection results in equivalent long-term control of regional disease, 2) determine whether sentinel node resection alone compared to axillary dissection plus sentinel node resection results in equivalent disease-free and overall survival, 3) determine the magnitude of morbidity reduction of sentinel node surgery versus axillary resection, 4) determine the magnitude of quality of life improvement by sentinel node surgery versus axillary node resection, 5) determine whether immunohistochemical analysis of H&E negative sentinel nodes identifies patients at risk for decreased overall and disease-free survival, and 6) establish a standardized method of sentinel node surgery in a large number of centers.