Objective To detect the expression of Krüppel like factor 8 (KLF8) in breast cancer tissues and cells and to explore the clinical significance of KLF8.Methods ① The Oncomine database was used to analyze the differential expression of KLF8 mRNA in the breast cancer tissues. The Kaplan-Meier Plotter database was used to analyze the relationship between KLF8 mRNA expression and prognosis (relapse free survival, overall survival, post-progression survival, and distant metastasis-free survival) of patients with breast cancer. ② The quantitative real-time PCR (qRT-PCR) and Western blot were used to detect the KLF8 expression levels in the 16 clinical patients with breast cancer and 7 breast cancer cell lines (MDA-MB-231, MCF-12A, Hs-578T, MCF-7, BT-474, MDA-MB-453, ZR-75-30) and normal breast epithelial cell lines MCF-10A, and the immunofluorescence was used to further detect the localization of KLF8 expression in the 2 breast cancer cell lines with higher KLF8 expression level. ③ The immunohistochemistry was used to detect the expression of KLF8 protein in 135 cases of breast cancer tissue microarrays, and the relationships between KLF8 protein expression and clinicopathologic characteristics or overall survival were analyzed.Results ① The Oncomine database showed that KLF8 mRNA expression in the breast cancer tissues was higher than that in the normal breast tissues (P<0.001). The median KLF8 mRNA expression level was taken as the cut-off point for high or low KLF8 expression. The results of Kaplan-Meier Plotter data analysis showed that the prognosis (relapse free survival, overall survival, postprogression survival, and distant metastasis-free survival) of patients with low KLF8 mRNA expression were better than those of patients with high KLF8 mRNA expression (P<0.05). ② The results of qRT-PCR and Western blot all showed that the KLF8 mRNA and protein expression levels in the breast cancer tissues were higher than those in the adjacent normal tissues (P=0.002, P<0.001). In addition, the Western blot results showed that the expression of KLF8 protein in the 7 breast cancer cell lines was higher than that in the normal breast epithelial cell lines MCF-10A respectively, and KLF8 protein mainly expressed in the cytoplasm of breast cancer cells and highly expressed in the nuclear of a few cells. ③ There were 63 cases of high KLF8 expression and 72 cases of low KLF8 expression by the immunohistochemical analysis of 135 patients with breast cancer tissue microarray (the H-score of the immunohistochemical test results was 75 as the cut-off point, H-score >75 was the high KLF8 expression and H-score ≤75 was the low KLF8 expression), the differences of statuses of estrogen receptor (ER) and progesterone receptor (PR) between the patient with high KLF8 expression and low KLF8 expression were significant (P<0.05). The Kaplan-Meier survival curve analysis showed that the prognosis of patients with high KLF8 expression was worse than that of patients with low KLF8 expression (P=0.002). The univariate analysis showed that the TNM stage, statuses of ER and PR, and KLF8 expression were related to the prognosis of patients with breast cancer (P<0.05), further multivariate Cox proportional hazards regression analysis indicated that the later stage of TNM and high KLF8 expression were the independent risk factors (P<0.05).Conclusions The results of this study suggest that KLF8 highly expresses in both breast cancer tissues and breast cancer cells, which is related to the statuses of ER and PR and prognosis of patients with breast cancer. KLF8 might be involved in the progression of breast cancer as an oncogenic gene, or it might provide a new direction for prognosis judgment and molecular targeted therapy of breast cancer.