MUC1 is a transmembrane molecule characterized by a repeated sequence of 20 amino acid (TAP PAHGVTSAPDTRPAPGS). Abnormal overexpression of MUC1 in cancer cells is thought to contribute to their aggressive growth, but molecular mechanisms associated with this effect are still unclear. Our current study aimed to clarify whether MUC1 expression as recognized by VU-3C6 anti-MUC1 mouse IgG monoclonal antibody (MAb) with a dominant epitope of 12 amino acids: GVTSAPDTRPAP, correlated with aggressive properties of human breast cancer. Immunohistochemical studies of 309 samples of formalin-fixed and paraffin-embedded materials showed no statistical correlation between MUC1 expression and clinicopathological parameters, as well as several breast cancer aggressiveness-related markers. Expression or nonexpression of MUC1 in 50 frozen samples, as determined by Western blotting, demonstrated no correlation with aggressive properties of breast cancer. However, samples with one MUC1-positive band more often had lymphatic vessel invasion and lymph node metastasis than those with more than two or three MUC1-positive bands (p<0.014 and p<0.043, respectively). Because VU-3C6 MAb recognizes MUC1 with short branches of O-glycosylated core carbohydrates, we used immunohistological methods to examine Tn antigen (precursor antigen: GalNAcalpha-O-Ser/Thr), Thomsen-Friedenreich (T) antigen, and sialyl-Tn antigen (STn) antigen. We found a strong correlation between expression of MUC1 and Tn antigen (p<0.0006), and samples with Tn antigen expression had more lymphatic metastasis than those with no such expression (p<0.08). We concluded that the lack of polymorphic MUC1 expression with Tn antigen is one characteristic related to aggressive breast cancer.