Abstract: Objective To investigate the expression of nuclear receptor coactivators AIB-1 and the relationship and clinical meaning with the clinical-pathological features in breast cancer. Methods The study included 218 cases with breast cancers, 45 cases of normal breast tissue and 45 cases with benign lesions of breast.Using immunohistochemistry, we examined the expression of AIB-1 in all cases. Patients' age, the state of menstruation, the size of tumors, clinical stages, pathological types, histological grades, the metastatic state of the axillary lymph nodes, ER, PR and HER-2 of all the breast cancer patients had been sorted to be analyzed. Results Among breast cancers(54.59％) ，benign lesions(33.33%) and normal breast tissues(22.22%), the overexpressions rate of AIB-1 protein in three groups above showed significant difference (P＝0.000) . The overexpression rate of AIB-1 protein in grade Ⅲ(63.39%) was higher than these in grade Ⅰ(18.18%) and in grade Ⅱ(48.42%) (P=0.004) . The overexpression rate of AIB-1 protein in the cases that the number of the metastatic axillary lymph nodes was more than three (73.47%), which was higher than the others (49.11%) (P=0.003) . In the cases with breast cancers,the overexpression rate of AIB-1 had no correlation with ER and PR(P＞0.05). But the overexpression rate of AIB-1 was higher in patients with HER-2(+)(64.08%) than that in patients with HER-2(-)(46.08%) (P=0.008). Conclusion In the cases with breast cancers, the overexpression of AIB-1 may correlate with the clinical phenotype and biology of breast cancer as well as poor prognosis.