PURPOSE: Galectin 3 is a beta-galactoside binding protein that is involved in various biological processes such as cell adhesion, migration, cell growth, tumor progression and metastasis. Although the precise acting mechanisms of Galectin 3 are unclear, it have been reported that the expression of Galectin 3 may related to tumor progression and metastasis. We investigated the immunohistochemical expression of Galectin 3 in 57 cases of benign and malignant breast neoplasm to evaluate the relation of a Galectin 3 expression to malignancy of breast neoplasm and the acting mechanism of Galectin 3. METHODS: Twenty fibroadenomas, 7 intraductal papillomas, 10 intraductal carcinomas and 20 invasive ductal carcinomas were studied. Immunostaining of Galectin 3 was evaluated in comparison with that of the internal controls, and the intensity of immunostaining was semiquantitatively graded on an intensity scale of 0 to 3. RESULTS: The normal ductal epithelium of the breast showed strong immunoreactivity with an intensity 2 to 3. The staining gradually and significantly decreased in accordance with the histopathological type and tumor progression from a fibroadenoma, intraductal papilloma, ductal carcinoma in situ and invasive ductal carcinoma (P<0.001). In particular, the expression of galection-3 was prominently decreased in invasive ductal carcinoma. CONCLUSION: The galectin-3 expression pattern suggests that progression from benign breast tumor to malignant breast tumor leads to a reduced expression of galectin-3.