Background: The objective of this study is to assess the oncologic safety of sentinel lymph node biopsy (SLNB), especially with regard to the axillary recurrence (AR) rate, and to determine the risk factors for AR and disease-free survival (DFS) and overall survival (OS) after negative SLNB. Materials and Methods: Between 2003 and 2006, a total of 1626 patients with invasive breast cancer and clinically axillary node-negative tumors underwent SLNB using a radioisotope at the Asan Medical Center. Of these patients, 1196 were negative on SLNB. Among these, 709 underwent SLNB only, and 487 underwent SLNB and axillary lymph node dissection (ALND). We included patients with any size tumors, except for those with inflammatory breast cancer, if patients had clinically negative lymph nodes. Results. Mean follow-up was 70.2 months for the SLNB-only group and 71.5 months for the SLNB and ALND group. The 5-year axillary-free survival rates were 98.91% (95% confidence interval [95% CI] 70.2-71.0) and 99.36% (95% CI 71.3-72.0), respectively; the 5-year DFS rates were 95.17% and 95.18%, respectively (log rank P =.543); and the 5-year OS rates were 98.36% and 98.75%, respectively (log rank P =.380). Univariate analysis showed that negative hormone receptor status (P =.002) and high tumor grade (P =.032) were significant prognostic factors for AR in the SLNB only group. Multifocality and tumor size did not affect the rate of AR. Conclusion: SLNB alone is an oncologically safe procedure in clinically node negative patients abrogating the need for further ALND. Negative hormone receptor status and high tumor grade might be risk factors for AR.