Purpose: Isosulfan blue has been traditionally used as a tracer to map the lymphatic system during identification of the sentinel lymph node (SLN). However, this vital dye is difficult to obtain in Korea. Radioisotopes such as technetiumlabeled sulfur colloid or albumin colloid are also expensive and complex to use. The purpose of this study is to evaluate usefulness of a sentinel lymph node biopsy (SLNB) using methylene blue dye in breast cancer patients. Methods: We evaluated the sentinel node mapping experience using methylene blue dye from July 2003 to January 2007. Fifty-eight patients with clinical T1-T2 breast cancer without palpable axillary lymph nodes were enrolled. All SLNs were submitted for intraoperative frozen section and hematoxyline and eosin (H & E) stain analysis. For the negative SLNs, serial sections of each SLN specimen were examined by permanent H & E staining and by immunohistochemical techniques (IHC) using cytokeratin. Regardless of the results of a frozen section for the SLNs, a backup level II or III axillary lymph node dissections (ALND) was performed. Results: Of the 58 patients that underwent a SLNB using methylene blue dye, an SLN was identified in 56 patients (96.6%), and metastatic SLNs were detected in 14 cases. Axillary lymph node metastasis found in 18 out of 58 patients. Thus, the false negative rate for a SLNB was 22.2% (4/18). Two patients had a micrometastasis (pN1mi) and two patients had clusters of isolated tumor cells (pN0[i+]) that were identified in the SLNs by IHC with the additional use of cytoketatin. The sensitivity, specificity, and accuracy of the SLNBs were 77.8%, 100%, and 92.9%, respectively. The false negative rate improved with the accumulation of experience for performing a SLNB (12.5% vs 30.0%). The sensitivity, specificity, positive predictive value, and accuracy of preoperative ultrasonography (USG) for an axillary lymph node metastsis were 50.0%, 95.5%, 81.8% and 81.0% respectively. Conclusion: Based on our initial experience, methylene blue dye is safe, inexpensive, and a readily available tracer for the SLN mapping, and it could be an effective alternative to the use of isosulfan blue dye for accurately identifying SLNs in early breast caner patients. We expected that the findings of preoperative USG could serve as useful adjuncts to a SLNB.