PURPOSE: CD24 is a small heavily glycosylated glycosylphosphatidylinositol-linked cell surface protein, which is expressed in hematological malignancies as well as a large variety of solid tumors. The authors aimed to evaluate the CD24 protein expression in fibroadenomas and adenocarcinomas of the breast and its correlation to clinicopathological data. METHODS: Immunohistochemical staining for CD24 was performed on 28 mammary neoplasia, diagnosed as either adenocarcinomas (22 cases) or fibroadenomas (6 cases), to examine the relationship with clinicopathological parameters. The results of the immunohistochemical staining were evaluated by the stainability (negative, weak-, moderate-, strong-positive) and staining patterns (membranous vs. intracytoplasmic) for statistical analyses. RESULTS: The present study clearly demonstrates that CD24 was abundantly expressed in adenocarcinoma, compared to in fibroadenomas of the breast (P<0.001). Intracytoplasmic staining was noted in the adenocarcinomas only but this was not statistically significant between the adenocarcinoma and fibroadenoma groups. No significant correlations of the CD24 stainability or staining pattern were detected with the nodal status, tumor histological grade or histological subtypes. CONCLUSION: Our data suggest that abundant membranous expression or intracytoplasmic expression of CD24, as detected by immunohistochemistry, is an important tissue marker for a mammary epithelial neoplasm, which could help to define adenocarcinomas from fibroadenomas.