OBJECTIVE: Cancer of the thyroid is a relatively rare malignant neoplasm with an increasing incidence in the last decades. Ionizing radiation is the best documented risk factor for thyroid malignant neoplasms, in current medical literature. An increase in the global annual per caput effective radiation dose has been described, partly as a result of medical exposure. External beam radiation therapy of the head and neck region has also been attributed. Thyroid tissue is particularly vulnerable to the carcinogenic effects of ionizing radiation because of its anatomical position and radiosensitivity. We review the relationship between external beam radiotherapy and secondary thyroid cancer and of factors influencing this correlation. Since 1950 this causal relationship has been confirmed by numerous researchers. The role of the primary disease, the administrated radiation dose, the irradiation field, sex, age and genetic background and also the time interval between radiation and the onset of thyroid cancer are the main factors that have been studied in medical literature. Developments in the field of radiotherapy, recent findings on the relationship between head and neck carcinomas and thyroid carcinomas and interactions between confounding factors, raise the need for further study on this subject.