According to the histological typing system of thyroid tumours adapted by the World Health Organisation follicular tumours are classified as follicular adenomas and follicular carcinomas. The most important diagnostic criterion for follicular carcinoma is the evidence of vascular and/or capsular infiltration. The aim of this work was to verify the Ag-NOR counting method for the diagnosis of thyroid tumours. From the material of the experiment on mice that drunk thyrostatic (1.2% NaCIO4) and were irradiated 10 histologically normal thyroids, 10 showing hypertrophy and hyperplasia, 8 follicular adenomas and 10 follicular carcinomas were used. Thyroid glands were impregnated with the silver to present Ag-NORs. The average numbers of Ag-NORs per nucleus (Ag-NOR/nucleus) for each group were determined. The results showed that the method of counting Ag-NOR can be used to differentiate among thyroid tumours.