BACKGROUND/AIMSThe new pathological prognostic staging in the 8th edition of the American Joint Committee on Cancer (AJCC) Cancer Staging Manual uses biomarkers, such as estrogen receptor (ER), progesterone receptor (PR), and human epidermal growth factor 2 (HER2) for breast cancer staging, but not Ki-67. This study was designed to evaluate the relationship of Ki-67 with pathological prognostic staging parameters and its possible correlation with this new staging system.MATERIAL and METHODSWe performed a retrospective analysis on 59 invasive ductal breast carcinoma patients. We restaged all the patients using anatomic staging (AS) and pathological prognostic staging (PPS). The correlation of Ki-67 with ER, PR, HER2, histological grade, tumor size, and lymph node status were compared using the Chi-square test.RESULTSWhen patients classified according to AS were restaged using PPS, 21 (36%) retained their original stage, while 34 (58%) were downstaged and 4 (6%) were upstaged. There was no correlation between the stage change and Ki-67, HER2, tumor grade, or size. Both, ER and PR positivity were markedly higher in the downstaged group (p=0.014 and p<0.001). Ki-67 was not significantly different between AS patients; however, stage 3 PPS patients had a significantly more positive Ki-67 ratio than stage-1 and stage-2 patients (p=0.007). Moreover, Ki-67 had a significant negative correlation with ER and PR and positive correlation with the tumor grade, HER2, and lymph node involvement.CONCLUSIONKi-67 is not useful for predicting the staging change from AS to PPS. However, it is strongly correlated with markers related to the biological features and prognosis in breast cancer. In order to increase its usefulness, more comprehensive studies are required.