Abstract: Objective To study the expression of Thr157 phosphorylated p27^kip1(P-p27Thr157) in human breast carcinoma and its clinical significance. Methods The expression of P-p27Thr157 protein was detected respectively by immunohistochemical EnVision method and Western blot in 111 specimens of breast carcinoma tissues and 8 paired specimens of breast carcinoma and adjacent normal tissues.The relationships between the expression levels of P-p27Thr157 protein and its clinicopathological characteristics in primary breast carcinoma were retrospectively analyzed. Results The positive expression rates of P-p27Thr157 protein was 87.4% (97/111),including high expression in 64 cases(57.7%,64/111),low expression in 47 cases(42.3%,47/111) and 9.9%(11/111) in cancerous and paired adjacent normal tissues respectively(P<0.05).Strong P-p27Thr157 expression was seen in cancerous tissues but not in paired adjacent normal tissues.P-p27Thr157 expression was correlated with tumor size,lymph node metastasis and TNM stage(all P<0.01).The prognosis of patients with high expression of P-p27Thr157 was significantly poorer than of those with low expression.Moreover,high expression of P-p27Thr157 appeared to be correlated with a shorter overall survival(OS) of breast cancer patients.P-p27Thr157 expression was positively correlated with proliferation index Ki-67(r=0.528,P<0.05),and negatively correlated with p27(r=-0.458,P<0.05). Conclusion The abnormal expression of P-p27Thr157 protein may be involved in the development and pathogenesis of breast carcinoma,and it might be used as a novel diagnostic marker and therapeutic target in breast carcinoma.