A total of 40 patients (mean age 51 yrs; 36-89 yrs) with clinically T1-T2(< 3 cm)N0M0 breast cancer underwent sentinel node (SN) mapping with radioactive tracer (99mTc) injection only in 21 patients, with Patent blue V in 1 patient, or with both techniques in 19 patients. The preoperative injection of 99mTc (20-40 MBq) was followed by lymphoscintigraphy. A handheld gamma probe was used to detect the SN in the operative room. A lumpectomy and an axillary dissection were performed in all the patients. SNs could be identified in 39/40 patients, resulting in a sensitivity of 98%. Successful localization of the SNs was accomplished by isotope only in 19/20 patients, by blue dye only in 1/1 patient, and by both methods in 19/19 patients; in 2 of these 19 patients, SNs were identified by blue dye only. Axillary metastases were found in 12/40 patients (30%), the SN being the only nodal metastasis in 8/12 patients (75%). Six of these 12 patients (50%) had only evidence of micrometastasis. Negative SNs on serial sections stained with hematoxylin-eosin (H&E) were evaluated with cytokeratin immunostain (C11). In all cases of negative SNs the remaining axillary nodes were also free of tumor, resulting in a negative predictive value of 100%. We conclude that SN mapping is a highly accurate method for staging the axillary node status in breast cancer patients. Optimal localization is achieved by the combination of injection of 99mTc-colloid and blue dye as evidence by the cases of positive SN identified by only one of both methods.