Histological material from 60 cases of follicular carcinoma of the thyroid operated between 1962 and 1987 was examined morphometrically to see whether this would produce data that could be correlated with survival. Nuclear parameters (area, perimeter, minimum and maximum diameter and their ratio, form factor) and the mitotic index were investigated. At the same time, account was taken of certain clinical parameters (sex, age, degree of differentiation, size of the primary lesion, extension of the tumour) potentially correlated with prognosis. Survival in relation to all these variables was examined by means of an actuarial method and subjected to both uni- and multivariate analysis. The results, although suggesting the existence of a relationship between nuclear size and the aggressiveness of this type of tumour, failed to demonstrate a significant correlation between these morphometric parameters and survival after treatment. By contrast, survival was significantly influenced by some of the clinical variables, particularly loco-regional extension of the disease and the patient's age. The prognostic significance of nuclear size was recently demonstrated in breast cancer and in a series of thyroid carcinomas including tumours with different morphologies and progression patterns. In follicular thyroid carcinoma the degree of cellular pleomorphism is usually low. So, morphometry does not seem able to provide information regarding prognosis more valuable than those obtained by the clinical data.