BACKGROUND Increased incidence of childhood thyroid carcinoma, particularly in the youngest children, has been reported from Belarus since the nuclear reactor accident at Chernobyl in 1986. The relation between disease severity and age at the time of the accident, not previously established in this cohort, was analyzed in this study. METHODS The authors studied the association between disease severity, expressed by TNM classification, and age at radiation exposure in a cohort of 483 patients younger than 8 years at the time of the Chernobyl accident who have been diagnosed with differentiated thyroid carcinoma since 1986 at the Center for Thyroid Cancer in Minsk. The associations between age at radiation exposure and TNM categories were compared among 4 groups of patients who were ages <2, 2.1-4, 4.1-6, and 6.1-8 years at the time of the accident. Multivariate discriminant analysis was performed to examine the effects of age at the time of the accident, gender, histology, tumor stage, and N classification on the frequency of distant metastasis. RESULTS Younger age at the time of the Chernobyl accident was associated with greater extrathyroidal tumor extension (P<0.01) and more lymph node involvement (P<0.0001) and tended to be associated with more distant metastases (P = 0.09). Compared with patients who were ages 6.1-8 years at the time of the accident, patients who were younger than 2 years had significantly more extrathyroidal tumor invasion (P = 0.004), lymph node involvement (P = 0.004), and distant metastases (P = 0.05). The age at diagnosis increased with older age at the time of radiation exposure (linear regression analysis; correlation coefficient = 0. 67; P<0.001). Multivariate analysis revealed that younger age at the time of the accident (P = 0.001) and advanced locoregional tumor extension (P<0.001) were the only powerful factors influencing the risk for distant metastasis of this malignancy. CONCLUSIONS The severity of disease was associated inversely with age at the time of radiation exposure in these cases of radiation-induced childhood thyroid carcinoma.