Background: The ACOSOG Z0011 study revealed that ALND can be omitted without changing prognosis in selected breast cancer patients even if they have SLN metastasis. However, determining the total number of positive axillary lymph nodes is still important for deciding on the postoperative treatment . To address this issue, we developed the National Cancer Center SLN metastatic score (NCS score) based on a detailed evaluation of SLNs in patients with breast cancer using a combination of histology and one-step nucleic acid amplification (OSNA) measuring CK19 mRNA . Methods: 1176 patients were diagnosed with primary breast cancer ( cTis–3 N0) at our institution based on a combination of histology and OSNA for SLNs. Of these, 323 patients who underwent additional ALND for positive SLN metastases were included in this study. Each SLN was divided and tested with histology and OSNA. Histology classified as macrometastasis, micrometastasis, and isolated tumor cells, which were assigned 3, 2, and 1 points, respectively. Whereas, OSNA classified as 2+, 1+, and +I, which were assigned 3, 2, and 1 points, respectively. In each patient, the sum of histology and OSNA points for all SLNs was defined as the NCS score. We compared the performance of the NCS score to other clinical factors in predicting N2 status ( ≥ 4 metastatic LNs ) and RFS using ROC curve analysis with the area under the ROC curve (AUC) and odds ratios (ORs). Results: The NCS score and OSNA result performed better on predicting N2 status (AUC = 0.920 and 0.824) than cT stage, pT stage, ly and pathology findings, respectively. NSC score and OSNA result were both significantly correlated with RFS in the 323 patients ( p = 0.0040 and p = 0.0026 ), suggesting that they are good prognostic factors for metastasis. In these 323 patients, NSC score and OSNA result were also significantly correlated with RFS in ER-negative patients ( p = 0.0241 and p = 0.0050) and triple-negative patients ( p = 0.0459 and p = 0.0132 ), indicating that they are exceptionally good prognostic factors for distant recurrence. Conclusions: NCS score and maximum OSNA copy number are factors for predicting number of metastatic lymph nodes, and prognosis.