Purposes: We investigated the prevalence of thyroid abnormalities and their follow-up features in childhood cancer survivors. Materials and Methods: Eighty of 150 childhood cancer survivors who elapsed at least 5 years after completion of cancer treatment were recruited. We investigated the prevalence of thyroid abnormalities by ultrasonographic findings and thyroid function tests (TFTs). We also analyzed their follow-up data from the survivors who visit long-term follow-up clinic every year. Results: The median age at diagnosis of primary cancer was 4 years (range,0-21 years); males were 60 (75%). Thyroid nodules were found in 13 patients (16.3 %). A median of 4 years (range, 0-21years) elapsed between the diagnosis of primary cancer and the discovery of thyroid nodule. Of 13 (16.3 %) patients who received head and neck radiation therapy, 3 (4 %) patients were detected. However, there was no significant change of thyroid lesions by repeated ultrasonography in 49 survivors who performed yearly at least 2 times. The incidence of thyroid nodules was not significantly associated with sex, irradiation, or age. Abnormal TFTs were revealed in 16 (20%) including 5 patients of overt hypothyroidism and 11 patients of subclinical hypothyroidism. Head and neck irradiation more than 3000 cGy were significant risk factors of persistent hypothyroidism. Conclusion: The prevalence of thyroid abnormalities detected by ultrasonography was higher compared to historical normal children group. In addition, head and neck irradiation was related with persistent hypothyroidism. Long-term follow-up of thyroid abnormalities and TFTs in CCSs would be needed.