The prognostic value of nuclear DNA content in follicular thyroid tumours was studied in 65 patients. Forty-six (20 with follicular carcinoma and 26 with follicular adenoma) were alive at least 10 years after diagnosis, and 19 had died of follicular thyroid carcinoma 2 months to 13 years after diagnosis. DNA was measured in morphologically identified single tumour cells either on material from fine-needle aspiration biopsy or on histologic sections from the primary tumours. Metastases or locally recurrent tumour were also examined in ten cases. The tumours from the surviving patients had cells with DNA content comparable to that in normal cells, whereas the tumours from the nonsurvivors showed higher DNA values. Primary tumours had essentially the same DNA content as metastases or local recurrence in eight of the ten studied cases. The data suggest that DNA measurement in follicular thyroid tumours offers a valuable addition to standard clinical and microscopic analysis.