Radioiodine treatment of hyperthyroid patients with autonomous thyroid nodule leads to cellular DNA damage not only in thyrocytes but also in peripheral blood lymphocytes. The purpose of this study was to evaluate DNA breakage and base damage in thyrocytes and lymphocytes in patients treated with 131-I. In all the patients thyroid scintiscan was performed using 131-I. Damage to DNA was estimated by comet assay. Samples were taken before radioiodine treatment, and 12 and 54 days afterwards. Our results indicate high diversity in the level of DNA damage among the individual patients. However, in all cases, after 54 days the level of DNA damage in lymphocytes was similar or even lower than that in the controls. In contrast, in hot nodule the DNA damage persisted until the 54th day after 131-I application. Differences in the type of DNA damage between thyrocytes and lymphocytes were also observed. In lymphocytes there was more base damage, whereas in thyrocytes single strand breaks prevailed. This may indicate different mechanisms of DNA damage induction and/or DNA repair.