Polo-like kinase 1 (PLK1) is one of the serine threonine kinases that contributes to cell mitosis and is regarded as a marker of cellular proliferation. However, its protein expression in human carcinoma has not been studied in depth. We investigated PLK1 expression in various thyroid neoplasms in order to elucidate its physiological significance in thyroid carcinoma. Normal follicular cells only occasionally expressed PLK1. In follicular tumours and anaplastic carcinoma, PLK1 overexpression was not a common event and only 5.9% of follicular adenoma, 7.1% of follicular carcinoma, and 11.8% of anaplastic carcinoma overexpressed this protein. However, 43.7% of papillary carcinoma overexpressed PLK1. Polo-like kinase 1 overexpression was more frequently observed in smaller papillary carcinoma lesions, and 62.5% of microcarcinoma (ranging from 4 mm to 1.0 cm) and even 66.7% of incidental carcinoma (less than 4 mm) overexpressed it, whereas this phenomenon could only be seen in 20.0% of lesions larger than 4.0 cm. Furthermore, PLK1 overexpression was not related to cell-proliferating activity evaluated by Ki-67 labelling index, but it was inversely linked to UICC stage, extrathyroidal invasion, and the presence of poorly differentiated lesion as proposed by Sakamoto et al. These findings strongly suggest that, unlike other carcinomas previously studied, PLK1 does not act as a cell cycle regulator but plays a constitutive role in papillary carcinoma especially in the early phase, and may contribute to the malignant transformation of this carcinoma.