Using Cox's Proportional Hazard Model, we have demonstrated the influence of age, sex, microscopic tumour type, extent of primary tumour, nodal status and the presence of metastases on prognosis, in our population of 441 patients with thyroid carcinoma. The TNM classification contributes significantly to survival, but does not include other contributory prognostic variables, whereas the prognostic index developed by the EORTC thyroid study group, which takes account of age and histology, proved a reliable predictor of survival for our patient group.