To determine whether the expression of Sialosyl-Tn antigen may have its putative relationship as an established or potentially useful clinicopathologic prognostic parameter in breast cancer, we evaluated 110 patients with invasive breast carcinoma, using formalin-fixed, paraffin-embedded tissue sections. STn antigen was detected by the HB-STn antibody using an avidin-biotin-peroxidase method. The parameters studied were tumor size, nodal status, stage, histologic grade, ER/PR expression, DNA ploidy and S-phase fraction. STn expression was observed in 42 patients (38.2%) with breast cancer. The expression rate of STn was associated with axillary node metastasis. A tendency towards an association between STn expression and tumor size or TNM stage. There was no relationship between STn expression and histologic grade, ER/PR expression, DNA ploidy and S-phase fraction. We conclude that the detection of STn expression may be useful for predicting lymph node metastasis.