Regional lymph node metastases in patients with breast cancer have fundamental staging, prognostic, and treatment implications. Classically, axillary lymph node sampling requires a dissection under general anesthesia. The concept that a primary, or sentinel, lymph node is the first node to receive drainage from a tumor has been established in patients with malignant melanomas using radiolabeled tracers and vital dyes. This study proposed two hypotheses: (1) radiolabeled sentinel lymph nodes can be identified in most patients with breast cancer, and (2) radiolabeled sentinel lymph node biopsy accurately predicts axillary lymph node metastases in those patients. Patients with operable breast cancer had Tc-99 sulphur colloid injected around their breast tumors 1-6 hours preoperatively. Patients underwent gamma probe identification of sentinel lymph nodes that were biopsied. All patients underwent axillary lymphadenectomy in conjunction with lumpectomy or mastectomy. Fifty female patients ages 26 to 90 years underwent lumpectomies with axillary dissections (40 patients) or modified radical mastectomies (10 patients). Sentinel lymph nodes were identified in 42 of 50 patients (84%). Eight patients (16%) had metastases to the axillary lymph nodes. In 7 patients, sentinel lymph nodes correctly predicted the status of the axillary nodes. There was one false negative result. A total of 550 lymph nodes were resected for an average of 11.2 nodes per patient. Sentinel lymph node scintigraphy and biopsy accurately predicted the axillary lymph node status in 41 of 42 patients (98%). Scintigraphy can identify sentinel lymph nodes in a large majority of patients. Sentinel lymph node biopsy is an accurate predictor of axillary lymphatic metastases.