CD24 is a small, heavily glycosylated cell surface protein, that is expressed in a large variety of solid tumors. It is considered to play an important role in tumor progression and metastasis. We aimed to evaluate CD24 expression in invasive ductal carcinomas (IDCa), ductal carcinoma in situ (DCIS) and non-tumorous breast tissues, and to investigate the relationship between histopathological parameters, estrogen and progesterone receptors, and c-erbB2 expressions. The study included 34 IDCa, 25 DCIS, and 13 non-tumorous breast tissues. All cases were reevaluated histopathologically, and immunohistochernistry was performed with monoclonal CD24 antibody. The results clearly demonstrated that CD24 expression, including membranous and cytoplasmic staining, was significantly higher in DCIS and IDCa than in the non-tumorous breast (p = 0.001, p = 0.000, and p = 0.035, p = 0.000, respectively). Cytoplasmic staining was detected predominantly in neoplastic tissues and was significantly increased in high grade DCIS (p = 0.0 13). In invasive carcinomas, although the level of membranous staining was significantly positively correlated with tumor grade.(p = 0.040), there was no such an association with the cytoplasmic level. However, it showed a trend towards pT (p = 0.089). In conclusion, our results suggest that higher CD24 expression may be associated with malignant transformation and progression in breast cancer biology. Furthermore, higher membranous expression and, in particular, cytoplasmic staining seem to predict malignant transformation, and different patterns of CD24 expression may be associated with different pathological features in breast tumors. (c) 2006 Elsevier GrnbH. All rights reserved.