A large increase in the incidence of thyroid cancer, especially in young children, has been confirmed with a short latent period after the Chernobyl accident. To elucidate the relationship of cause-and-effect and the mechanism of radiation- induced thyroid cancer, several approaches have been performed and clinical characteristics have been analyzed. The incidence of thyroid cancer in children was still high, and clinical characteristics have been analyzed. By the molecular biologic studies, new types of ret/PTC gene rearrangement have been found and might be involved in an etiology of post- Chernobyl thyroid cancer. The prevalence of thyroid cancer in adult liquidators has also been reported; however, no clear increase of thyroid cancer has been observed yet. Thyroid diseases other than thyroid cancer around Chernobyl were also investigated. Prevalence of goiter and urinary iodine excretion levels in children around Chernobyl were reported. Relationships between autoimmune thyroid disorders and radiation are also other topics. The recent data on post-Chernobyl thyroid cancer support the direct evidence of radiation effect on children; however, further analysis, such as follow-up study, is needed.