To monitor the extent and the duration of lymphocyte subset changes in patients with thyroid carcinoma undergoing therapeutic 131I administration, the percentage of lymphocyte subsets were serially analyzed before and after 131I treatment. In patients who received 1850 MBq of 131I for ablation of thyroid remnants, only for NK cells and B cells showed a significant reduction. In patients received 3700 MBq of 131I for treatment of local lymph node me-tastases, NK cells, B cells and CD4+ were found decreased. In patients received 7400 MBq of 131I for treatment of distant metastases, NK cells, B cells and CD4+ and CD8+ were all affected. However, there is no significant reduc-tion compared to the baseline in the percentage of all lymphocyte subsets three months after 131I treatment. The re-sults show that the sensitivity of lymphocytes to 131I internal radiation depends upon lymphocyte phenotype and 131I activity. The immunosuppression effects are temporary and reversible. To monitor the extent and the duration of lymphocyte subset changes in patients with thyroid carcinoma undergoing therapeutic 131I administration, the percentage of lymphocyte subsets were serially analyzed before and after 131I treatment. In patients who received 1850 MBq of 131I for ablation of thyroid remnants, only for NK cells and B cells showed a significant reduction. In patients received 3700 MBq of 131I for treatment of local lymph node me-tastases, NK cells, B cells and CD4 + were found decreased. metastases, NK cells, B cells and CD4 + and CD8 + were all affected. However, there is no significant reduc tion compared to the baseline in the percentage of all lymphocyte subsets three months after 131 I treatment. The re-sults show that the sensitivity of lymphocytes to 131I internal radiation depends upon lymphocyte phenotype and 131I activity. The immunosuppression effects are temporary and reversible.