Thyroid cancer is a relatively rare cancer (5 new cases/y/10(5) inhabitants). An excess of thyroid carcinoma has been found in some but not in all goiter endemic areas. Follicular and anaplastic carcinomas have been found particularly frequent in regions of goitre endemia. A significant increase of thyroid carcinoma has also been found in iodine sufficient areas (Norway, Iceland, Hawaii). In several surveys a positive correlation has been found between parity and incidence of differentiated thyroid carcinoma. Natural goitrogens and chemotherapeutic agents have been proved to induce hyperplasia but their role in carcinogenesis of exposed populations is not yet definitely ascertained. Exposure to external radiation is carcinogenic for the thyroid both in human and in experimental animals. Patients treated for hyperthyroidism or thyroid cancer or given diagnostic doses of 131-I (0.5 Gy/test) indicate that under these conditions 131-I is not carcinogenic. Findings on population exposed to radioactive fallout showed an increased incidence of thyroid carcinomas compared to unexposed populations. After the Chernobyl accident (1986) particular attention was given to calculate the risk of thyroid cancer caused by the fallout of 131-I. Up to now a considerable increase of thyroid carcinoma has been reported in children of a region near Chernobyl (Belarus).