Several pilot studies have indicated that SLN biopsy can be used to identify axillary lymph node metastases in patients with breast cancer. To confirm this finding, a multicenter study in a variety of practice settings was performed. A total of 674 patients with breast cancer at five institutions were enrolled. The techniques of SLN identification included the vital dye-guided and the vital dye- and gamma probe-guided methods. The SLN was removed, and complete axillary lymph node dissection (ALND) was performed. SLN and ALND specimens were examined separately. The SLN was successfully identified in 214 (94%) of 227 patients using the combined dye- and gamma probe-guided methods. The SLN was identified in 332 (74%) of 447 patients using vital dye-guided method alone. Patient age of at least 21 years, medially located primary tumor, and clinically positive nodes were correlated with failure to identify the SLN. The accuracy of SLN biopsy for the detection of metastatic disease was 96% (522 of 546), and the sensitivity was 90% (203 of 226). Accuracy of 100% was achieved in the patients with tumors less than 1.6 cm in diameter. All 23 false negative results occurred with larger primary tumors. SLN biopsy can accurately predict the presence or absence of axillary lymph node metastases, particularly in patients with small (< or = 1.5 cm) breast cancers.