To improve our understanding of the basic biological mechanisms of Hürthle cell tumours (HCT), proliferative activity in 20 HCT was determined by Ag-NOR (nucleolar organiser regions) counting and by immunohistochemical markers proliferating cell nuclear antigen (PCNA) and MIB-1. Ten surgically removed Hürthle cell adenomas (HCA) and 10 Hürthle cell carcinomas (HCC) were used for the study. For comparison, a group of 10 thyroid adenomas without Hürthle cells (SA) was also evaluated. There were significant differences in the percentages of PCNA- and MIB-1-positive nuclear areas between HCA and HCC as well as between SA and HCA. The mean number of Ag-NORs per nucleus showed no significant differences between our groups. However, determination of percentages with 5 or more Ag-NORs per nucleus within one focal plane of sections was more useful for the differentiation between thyroid tumours included in our study. The combination of three types of cell kinetic markers may provide insight into the still poorly understood relationships among karyological abnormalities, enhanced proliferation speed, clonal expansion and invasiveness of HCT.