In this study, we demonstrated that (1) follicular cells in normal follicles only occasionally expressed PLK1, (2) the incidence of PLK1 overexpression was not high in either follicular tumours or anaplastic carcinoma, and (3) in papillary carcinoma, PLK1 overexpression was more frequently observed in cases showing low biologically aggressive phenotypes, although it was not related to cell-proliferating activity. Here, PLK1 is linked to undifferentiated carcinoma.