BACKGROUND: Abnormal glycosylation patterns have been recognized as a featureof carcinomaassociated mucins. The expression of the Tn antigen in breast cancer tissue was investigated to assess its prognostic relevance. METHODS: Formalin-fixed, paraffin-embedded materials from 219 patients with breast cancer were used. Immunohistochemical staining of the Tn antigen was retrospectively investigated and a lesion staining 10% or more was considered positive RESULTS: Tn antigen expression was present in 99 (45%) of 219 lesions. There were no correlations between Tn antigen expression and mean patient age, nodal status, estrogen receptor status, or menopausal status, but there was a slightly significant association between Tn and tumor size. Patients negative for the Tn antigen had a significantly better survival rate than those who were positive. Multivariate analysis also indicated that Tn expression correlated significantly with overall survival in addition to nodal status and tumor size. CONCLUSION: Tn expression was a significant prognostic factor in breast cancer, but the significance was lost on multivariate analysis. The biological implication of Tn expression in breast cancer needs further investigation.