BACKGROUND : The prognostic significance of tumor-infiltrating lymphocytes (TILs) has been established in breast cancers with estrogen receptor (ER)-negative and human epithelial growth factor receptor 2 (HER2)-negative or HER2-positive subtypes; however, its utility concerning the ER + /HER2 - subtype remains unclear. METHODS : We evaluated the prognostic value of TILs by analyzing 717 invasive breast cancer operation cases. TILs were classified into three groups based on the proportion of area within the tumor: low ( < 10%), intermediate (10-50%), and high ( > 50%). Disease-free survival (DFS) and overall survival (OS) were calculated according to TIL levels. RESULTS : Although there was no significant association between TIL levels and DFS or OS in all patients, high TILs were significantly associated with favorable DFS in Ki67-high (n = 238, p = 0.035) but not in Ki67-low (n = 470, p = 0.46) breast cancers. Multivariable analysis showed that high TILs were a significant and independent factor for DFS (HR 0.34; 95% CI 0.10-0.87; p = 0.023) among the Ki67-high group. In the ER + /HER2 - subtype, high-TILs showed favorable DFS in the Ki67-high group, although this was not statistically significant (p = 0.48); in contrast, unfavorable DFS was observed in the Ki67-low group (p = 0.027). CONCLUSIONS : In Ki67-high breast cancers, high TILs were associated with favorable DFS, irrespective of subtype, but increasing TIL levels correlated with worse DFS in the Ki67-low group with the ER + /HER2 - subtype. These results highlight variation in TIL prognostic significance between Ki67-high and -low breast cancers, particularly for the ER + /HER2 - subtype.