To evaluate the genotoxic effect of 131I, lymphocytes from 9 patients who underwent large dose (150 mCi) 131I therapy after total thyroidectomy were studied for sister chromatid exchanges (SCE) before and after their first radioiodine treatments. Frequency of SCE (FSCE) was counted in chromosomes of 30 lymphocytes in each patients, and was expressed as numbers of SCE per cell. Numbers of leukocytes were also observed during 131I therapy. Pretreatment FSCE (4.2±0.7) was not different from the control (3.8±0.4, p=0.17). However, the frequency was significantly elevated after 131I administration (at the second day, 7.9±0.8, p〈0.001) and was diminished but still significantly elevated after a week (at 9th day, 6.4±0.6, p〈0.001). While counts of leukocytes in the peripheral blood showed no change(p〉0.05). In conclusion, chromatids of human lymphocytes were significantly damaged after 131I treatment without any bone marrow supression. And the repair of SCE was not complete within one week.