The expression of proliferative cell nuclear antigen (PCNA) in follicular tumours of the thyroid was examined by immunohistochemistry. Both usual nonoxyphilic cell follicular tumours (non-OCT) and oxyphilic cell tumours (OCT) were subdivided into benign, indeterminate, encapsulated carcinoma, and widely invasive carcinoma types. Among non-OCT the percentages of PCNA-positive cells in benign tumours, encapsulated carcinomas, and widely invasive carcinomas was 2.5%-8.6%, 11.8%-39.1%, and 18.6%-20.0%, respectively. There was a statistically significant difference between benign tumours and encapsulated or widely invasive carcinomas, as in previous studies. A value of 10% was appropriate to distinguish benign from malignant lesions. PCNA-positive ceils in indeterminate-type non-OCT were not significantly different from those in benign tumours, ranging from 4.3%-19.6%, and occurring at more than 10% in three of six tuours. Among OCT the positivity was less than 10% in benign tumours (4.5%-7.8%) and more than 10% in malignant tumours (14.1%-35.9%) and all the eight indeterminate tumours (12.5%-27.3%), with a statistically significant differences between the benign tumour and each of the latter types. These results indicate that the examination of PCNA is valuable in diagnosis of thyroid follicular tumours and that the use of similar diagnostic criteria may be warranted in both non-OCT and OCT.