PURPOSE: False negative results obtained with the use of a sentinel lymph node biopsy (SLNB) can result in down staging of tumors, whereas the use of a more elaborated pathological examination of sentinel lymph nodes might lead to upstaging of tumors. The purpose of this study was to compare results after performing only an SLNB as compared with performing conventional axillary lymph node dissection (ALND) without an SLNB in pathologically node negative (pN0) breast cancer patients. METHODS: From April 2004 to June 2007, SLNBs were performed for patients with primary breast cancer who had no clinical evidence of a lymph node metastasis. A total of 272 patients were treated with only an SLNB. During the same period, 278 patients were confirmed as pN0 after conventional ALND without an SLNB. A prospectively collected database and medical records of these patients were reviewed. RESULTS: For patients that had undergone only an SLNB, there was no local or regional recurrence. A distant metastasis developed in four patients (1.5%). In patients that had undergone ALND without an SLNB, a recurrence was found in 13 patients (4.7%). Patients that had undergone only an SLNB showed significantly better disease-free survival as compared to patients that had undergone ALND without an SLNB (p=0.032). CONCLUSION: pN0 patients treated with only an SLNB showed a significantly better outcome as compared to patients treated with conventional ALND without an SLNB. These results suggest that performing an SLNB might result in the upstaging of a subset of patients who would have been understaged by the use of conventional ALND.