To evaluate whether thallium-201 scan can reflect tumour proliferative activity in thyroid nodules. We compared the degree of 201Tl uptake with the tumour proliferative ability as assessed immunohistochmically by the labelling index of proliferating cell nuclear antigen (PCNA) in malignant and benign thyroid nodules. The case material comprised ten benign and 31 malignant surgically resected nodules from a total of 41 patients. 201Tl scan was performed 5 min (early scan) and 2h (delayed scan) after intravenous injection of 74 MBq of 201Tl. The degree of 201Tl uptake was visually divided into three grades [from (-) to (++)], as compared with its uptake in normal adjacent thyroid tissue. Immunohistochemical staining of PCNA was performed using a monoclonal antibody for PC10 on paraffin-embedded specimens. On both the early and the delayed scans, the mean PCNA index in the nodules with an intense 201Tl, i.e. (++), was significantly higher than that in nodules with a lower or with negative 201Tl uptake. The correlation was higher on the delayed 201Tl scan (P=0.009) than on the early scan (P=0.019). Our results indicate that 201Tl uptake may reflect the tumour proliferative activity of thyroid nodules, and this is especially true with regard to the uptake on delayed scans.