PURPOSE: Mucins are members of the glycoprotein family expressed in benign and malignant epithelial cells. The aim of this study is to evaluate the relationships between the expression of mucins in breast ductal carcinoma and clinicopathologic parameters. METHODS: We constructed tumor microarrays based on 240 cases of invasive ductal carcinoma and 40 cases of ductal carcinoma in situ (DCIS) using formalin fixed, paraffin embedded tissues. We examined the expressions of MUC1, MUC2, MUC5AC, and MUC6 by immunohistochemistry. RESULTS: MUC1 demonstrated cytoplasmic, membranous, apical, and combinative expressions. Other mucins demonstrated cytoplasmic expression. In invasive ductal carcinoma, MUC1, MUC2, MUC5AC, and MUC6 were expressed in 93.6%, 6.2%, 4.8%, and 12.4% of cases, respectively; these rates were slightly, but not significantly, higher than observed in cases of DCIS. MUC1 expression was associated with estrogen receptor (ER) expression and negative MUC1 expression was associated with triple negativity. MUC6 expression was correlated with higher histologic grade, lymphatic invasion, lymph node metastasis, and HER2 positivity. No associations with any other clinicopathologic parameters were observed. CONCLUSION: Most invasive ductal carcinomas of the breast express MUC1, and this expression is associated with ER expression. MUC6 expression is correlated with some clinicopathologic parameters that are indicators of poor prognosis. To evaluate the role of MUC6 as a potential biomarker, further studies are warranted.