BACKGROUND: In breast cancer, the single most important prognostic indicator is the axillary nodal status of the metastasis. The sentinel lymph nodes are the lymph nodes of the primary tumor that is drained first, which can predict the presence of the tumor metastasis with high accuracy. This study was designed to find out how accurately the sentinel node biopsy using radioactive material can predict axillary nodal status. METHODS: The patients were selected from the department of surgery at Yonsei University Yongdong Severance hospital, consisting of 54 patients with breast cancer diagnosed as clinical stages I or II from Nov. 1998 to May, 1999. We performed lymphoscintigraphy and gamma-probe method with 99mTc-antimony trisulfide colloid for detection of sentinel nodes. The radiotracer was injected around the tumor 2 hours prior to the operation. In the operating room, after the sentinel node biopsy had been performed, levels II and II axillary lymph node dissection were completed. The sentinel node was divided into 3 pieces evenly. One piece was tested for frozen section diagnoses and the others were fixed in 10% formalin for H&E staining and immunohistochemistry(IHC) using cytokeratin antibody to detect metastatic tumor cells. RESULTS: The mean size of the primary tumor was 2.51cm in diameter and the mean number of the sentinel nodes was 1.59. The detection rate of the sentinel node was 94.4%(51/54). Among 35 patients with negative sentinel nodes on frozen section, 4 patients(11.4%) had tumor metastasis on both permanent serial H&E sections and IHC using cytokeratin. Out of the 31 patients with negative sentinel node by permanent serial section, 4 patients had tumor metastasis in nonsentinel axillary nodes. These results revealed that the sensitivity of the sentinel node biopsy was 83.3% and the negative predictive value to predict the axillary node status was 81.1%. The false negative rate of the sentinel node was 16.7% by the permanent H&E and IHC section and 33.3% by the frozen section. Especially, in casese with tumors loss than 2cm, the sunsitivity was 100% in frozen section as well as permanent section. CONCLUSION: A combination of preoperative lymphoscintigraphy and gamma probe-guided methods achieve high success rates in the identification of sentinel nodes. Multiple serial sections and IHC of the nodes are required to detect the micrometastasis of the sentinel nodes. The sentinel node biopsy is significantly accurate and may be considered as an alternative to replace the routine axillary node dissection, especially in T1 breast cancer patients.