This study aimed to develop a clinically practical immunohistochemistry assay to distinguish luminal B from luminal A breast cancer subtypes and investigate its prognostic value. Gene expression profiling classified 28% of tumors as luminal A and 19% as luminal B. The optimal Ki67 index cut point to distinguish luminal B from luminal A was determined to be 13.25%. In an independent cohort of 4046 breast cancers, the Ki67 index, combined with HER2 status and hormone receptor status, effectively subtyped tumors into luminal A, luminal B, and luminal-HER2 positive subtypes. Luminal B and luminal-HER2-positive tumors were associated with poor recurrence-free and disease-specific survival, even among patients treated with tamoxifen alone. The study concluded that the Ki67 index, along with ER, PR, and HER2 status, can effectively distinguish luminal B from luminal A breast cancers and provide valuable prognostic information.This study aimed to develop a clinically practical immunohistochemistry assay to distinguish luminal B from luminal A breast cancer subtypes and investigate its prognostic value. Gene expression profiling classified 28% of tumors as luminal A and 19% as luminal B. The optimal Ki67 index cut point to distinguish luminal B from luminal A was determined to be 13.25%. In an independent cohort of 4046 breast cancers, the Ki67 index, combined with HER2 status and hormone receptor status, effectively subtyped tumors into luminal A, luminal B, and luminal-HER2 positive subtypes. Luminal B and luminal-HER2-positive tumors were associated with poor recurrence-free and disease-specific survival, even among patients treated with tamoxifen alone. The study concluded that the Ki67 index, along with ER, PR, and HER2 status, can effectively distinguish luminal B from luminal A breast cancers and provide valuable prognostic information.