Polysialic acid (PSA) is highly expressed during embryonic development, but barely expressed during postnatal development, and may be 're-expressed' in cancer tissues. In this study, motility and migration assays were performed to compare the changes in cell behavior between non-malignant and maligant cells. Next, the expression levels of PSA were evaluated in 4 human and mouse normal breast or breast cancer (BC) cell lines using 1,2-diamino-4,5-methylenedioxybenzene-labeling HPLC technology, as well as in human clinical BC tissue samples. PSA expression was significantly higher in malignant cells (where it appeared to facilitate cell migration and motility) than in non-malignant cells. Enhanced PSA expression levels were also observed during epithelial-mesenchymal transition (EMT), a leading cause of cancer cell metastasis, which was induced in the NMuMG and MCF10A cells by treatment with transforming growth factor-β1 (TGF-β1). An increased PSA expression also correlated with the disease stage in the patients with BC (P<0.0001). Using RT-qPCR, we found that polysialyltransferase ST8SiaIV (PST) and polysialyltransferase ST8SiaII (STX), which are responsible for PSA synthesis, were differently expressed in the tested BC samples. However, PST, but not STX, was re-expressed in 14 out of 20 clinical BC samples. The findings of the present study indicate that the pathophysiology of BC involves the aberrant regulation of PSA expression and PST gene expression.