Physical and cytogenetic estimates of the whole-body radiation doses have been compared in 11 patients receiving large doses of iodine-131 for the treatment of thyroid carcinoma. The physical estimate was based on the measurement of thyroid uptake, of the plasma activity variation, and of urinary activity. The cytogenetic estimate was obtained from the analysis of chromosome aberrations in peripheral blood lymphocytes. Good agreement between the estimates was observed in patients whose thyroid glands had previously been ablated by radioiodine. In patients who had varying degrees of thyroid function, there were considerable differences between the estimates with the cytogenetic value always being higher. It is suggested that these differences might be due in part to non-uniform irradiation of lymphocytes by local sources of activity in the thyroid and in the liver.