The aim of the current study was to investigate the expression of the proliferation antigen, Ki67, in triple-negative breast cancer (TNBC) and its correlation with clinicopathological factors. The expression of Ki67 and other biological indicators in 24 cases of TNBC tissues and 178 cases of non-TNBC tissues were detected using immunohistochemistry. Their correlation with the clinicopathological factors were also analyzed using the χ2 test. The positive rate of Ki67 expression in TNBC tissues was 83.3%, exhibiting a statistically significant difference when compared with that in non-TNBC tissues (73.0%) (P<0.05). The expression of Ki67 in breast cancer tissue significantly correlated with the tumor size and lymph node metastases; however, no correlation was observed with the age and the clinical stage. Ki67 may be an indicator of poor prognosis in TNBC patients.