BACKGROUND: Sentinel lymph node (SLN) biopsy has been the standard modality for breast cancer patients with clinically node negative disease. In patients who undergo axillary lymph node dissection (ALND) due to SLN metastasis, the harvested nodes (non-SLNs) often contain no metastasis. Here, we evaluated the predictive factors associated with non-SLN metastasis in breast cancer patients. MATERIALS AND METHODS: This was a retrospective study of patients with operable cT1-3, cN0 invasive breast cancer who underwent SLN biopsy followed by ALND due to SLN metastasis. The clinicopathologic factors and predictive factors of non-SLN metastasis were analyzed. The optimal cutoff for the Ki67 index and the number of positive and negative SLNs that were predictive of non-SLN metastasis were evaluated using receiver operating characteristic curves. RESULTS: The median number of SLN and non-SLN was 3 and 11, respectively. Of the 150 patients, 52 (35.0%) had metastases in non-SLNs. The optimal cutoffs for the Ki67 index and the number of positive and negative SLNs were of 12%, 2, and 1, respectively. In the univariate analysis, the Ki67 index and the number of positive SLNs≥2 and negative SLNs≤1 were higher in the non-SLN + group than that in the non-SLN - group. The number of negative SLNs was as a predictive factor for non-SLNs metastasis in the multivariate analysis. CONCLUSIONS: The number of negative SLNs predicts the risk of non-SLN metastasis in breast cancer. When deciding on whether to omit ALND, the number of positive and negative SLNs should be considered.