The prognostic value of nuclear DNA content in Hürthle cell tumours of the thyroid was studied in 23 patients with more than 10 years follow up. Eleven of these neoplasms were classified as Hürthle cell carcinoma and 12 as adenoma. DNA measurements in morphologically identified single tumour cells were performed either on fine needle aspiration biopsy material or on histological sections from the primary tumours. The nuclear DNA content identifies those patients with a good versus a bad prognosis. These results correlate well with the findings in earlier studies about papillary, follicular and medullary thyroid tumours.