A follow-up study of chromosome aberrations in peripheral blood lymphocytes was carried out on 14 hyper thyroid patients treated with different doses of 131I (4 to 11 mCi) orally. The results obtained 24 hours to one year after treatment showed that the frequencies of chromosome aberrations in these patients were significantly greater than those before treatment (t>1.96, p<0.05). But during the post-treatment period the levels of chromosome aberrations determined at 8 separate examinations displayed no significant difference (t<1.96, p>0.05). In comparison with external irradiation, the patients treated with 131I had the same types but quite different statistical distributions of chromosome aberrations in lymphocytes. The chromosome aberrations induced by 131I deviated from Poisson's distribution significantly.