Over 200 cases of juvenile thyroid cancer have been found in Fukushima since 2011. This phenomenon is now regarded to be caused by overdiagnosis of self-limiting thyroid cancer which exists at high prevalence in the young. The overdiagnosis was caused by the thyroid screening with ultrasonography (US), which also caused overdiagnosis in Korea. For many reasons, the thyroid screening program in Fukushima is still ongoing without any changes. The majority of residents in Fukushima still request to continue the program since they are not informed about the harms of this program. Since the US examination is performed as one of the school program, the rate of participation by schoolchildren is over 90%, which has resulted in expansion of the disaster. The Fukushima thyroid cancer disaster provides us a valuable opportunity to reconsider about medical ethics.