Among 45 patients with medullary thyroid carcinoma, clinical parameters (age, sex, stage) and morphological findings (histological and immunocytochemical pattern, DNA content) were correlated with follow-up data. Prognosis was found not to be related to histological features or to immunoreactivity for a variety of hormonal and nonhormonal products typical of neoplastic C-cells. Instead, survival was significantly correlated to age, sex and stage of disease. The best prognosis was seen in women less than 40 years of age and revealing an early stage of disease. In addition, DNA parameters and the degree of aberrant epithelial Leu-M1 expression appeared to correspond to the outcome of disease in the majority of cases. A benign clinical course was twice as frequent among tumours of normal diploid DNA value compared to carcinomas of higher DNA content. Irrespective of other morphological and clinical features, local recurrences occurred 3 times and death resulting from tumour occurred 4.5 times more frequently among cases with marked Leu-M1 positivity in comparison to tumours with only slight or absent reactivity.