Various microscopic classifications of metastatic sentinel lymph nodes (SLN) have been reported along with predictors of additional lymph node positivity and their correlations with the prognosis. The purpose of this study was to re-evaluate these classifications in the Japanese population. We selected the following three classifications, based on the procedural simplicity of the measurements: maximum diameter (maximum diameter of the largest tumor lesion in the SLN; <0.1, 0.1-1.0, >1.0 mm), invasion depth (depth of tumor invasion measured from the capsule in the SLN; SI ≤ 0.3 mm, SII >0.3 to ≤ 1.0 mm, SIII >1.0 mm), and microanatomic location (microanatomic location of the tumor deposits within the SLN; "subcapsular", "parenchymal", "combined", "multifocal", "extensive"). A retrospective study, using prescribed survey forms, was carried out. Among the 450 patients, including the 149 cases with SLN metastasis, an additional lymph node positivity rate of 0% could be predicted only in patients with a maximum diameter category of less than 0.1 mm. As compared with that in the SLN metastasis-negative cases, however, the prognosis was poorer in cases with SLN metastasis, even those with lesions falling under the maximum diameter category of less than 0.1 mm, invasion depth category of SI (≤ 0.3 mm) and microanatomic location category of subcapsular. The prognosis is particularly poor for the microanatomic location category of extensive, which should thus be regarded as a macrometastasis. A prospective study with standardized procedures, including pathological evaluation, is needed in order to confirm our conclusion.