Background: The prognostic value of the Ki67 expression level is yet unclear in breast cancer. The aim of this study was to investigate the association between Ki67 expression levels and prognostic factors such as grade, Her2 and hormone receptor expression status in breast cancers. Materials and Methods: Clinical and pathological features of the patients with breast cancer were retreived from the hospital records. Results: In this study, 163 patients with breast cancer were analyzed, with a mean age of 53.4 +/- 12.2 years. Median Ki67 positivity was 20% and Ki67-high tumors were significantly associated with high grade (p<0.001), lymphovascular invasion (p=0.001), estrogen receptor (ER) negativity (p=0.035), Her2 positivity (p=0.001), advanced stage (p<0.001) and lymph node positivity (p<0.003). Lower Ki67 levels were significantly associated with longer median relapse-free and overall survival compared to those of higher Ki67 levels. Conclusions: High Ki67 expression is associated with ER negativity, Her2 positivity, higher grade and axillary lymph node involvement in breast cancers. The level of Ki67 expression is a prognostic factor predicting relapse-free and overall survival in breast cancer patients.