We have recently reported on a technique of gamma probe localization of radiolabelled lymph nodes to identify the sentinel node in malignant melanoma. In order to determine whether this technique is applicable to assist in staging breast cancer, a pilot study was begun to address two questions: (i) can the sentinel lymph node draining a breast cancer be identified for selective resection; and (ii) is the sentinel lymph node predictive of the status of the entire axillary lymph nodes? One to four hours prior to axillary lymph node dissection, 22 consecutive patients had approximately 0.4 mCi of technetium sulfur colloid in 0.5 ml saline injected around the perimeter of the breast lesion. A hand-held gamma counter was used at surgery to locate the lymph node(s) receiving drainage from the breast. A sentinel lymph node was identified in 18 of 22 patients. Of these 18 patients, the sentinel lymph node was positive in seven of seven patients, with pathologically verified metastatic breast cancer to at least one lymph node. In three out of seven patients, the sentinel lymph node was the only lymph node with metastatic cancer. In this pilot study of breast cancer patients, we conclude that: (i) radiolocalization and selective resection of sentinel lymph nodes is possible; and (ii) the sentinel lymph node appears to predict correctly the status of the remaining axilla. These data justify a larger clinical trial to verify the value of this technique.