Thyroid cancer was the first solid tumor that showed an increased incidence among the Japanese A-bomb survivors and recently published data indicated an increase of thyroid cancer among children in Belarus. The annual thyroid cancer rate between 1986 and 1989 was 4 cases and 2 years later a 14-fold increase was found. That study has several methodological weaknesses but is nevertheless alarming. The present paper reviews the current epidemiological knowledge on radiation-induced thyroid cancer, and discusses the methodological difficulties. It is concluded that low doses of brief gamma radiation induce thyroid cancer in juveniles. No study has yet proven a relationship between protracted low dose exposure and thyroid cancer or an increased thyroid cancer risk among adults after exposure to any form of ionizing radiation. Thyroid cancer seems to have a somewhat shorter latency period than other solid tumors and the dose-response relationship seems to be linear. The most important issues in radiation protection concerning thyroid cancer are the risk of a thyroid cancer following low dose and/or protracted exposure to ionizing radiation and following 131I exposure in childhood.