PURPOSE: Sentinel lymph node biopsy has emerged as a new standard method of treatment in small-sized breast cancer and has allowed the elimination of unnecessary axillary lymph node dissection in patients with negative axillary lymph nodes metastasis. In sentinel lymph node biopsy, pathologic analysis can be scrutinized and accurate stage-diagnosis can be made with the methods of serial sectioning and immunohistochemical staining on routine basis. In this study, we tried to evaluate the accuracy of sentinel lymph node biopsy. METHODS: We evaluated 500 cases of sentinel lymph node biopsy operated at Samsung Medical Center between October 1995 and February 2004. Cases were categorized in 2 groups: cases operated during the period of learning curve (October 1995~December 1999) and the period of clinical application (January 2000~February 2004). RESULTS: Overall failure rate and false negative rate of sentinel node biopsy were 9% and 14.7% respectively. The failure rate of the period of learning curve and the period of clinical application were 15.3% and 5.6% respectively (P <0.01). In sentinel lymph node detection, combined blue dye and radioisotope method was superior to blue dye or radioisotope method. About 40% of lymph node metastases were found in patients with tumors more than 2 cm in size. Clinical findings including previous biopsy, multiple tumor, neo- adjuvant chemotherapy, obesity and old age were not absolute contraindications for sentinel node biopsy. CONCLUSION: Sentinel lymph node biopsy can be an accurate and reliable method to examine the status of axillary nodes in a small breast cancer.