AIM: The aim of the work was to estimate the strength of influence of the first (131)I activity on the outcome of the first course of radioiodine treatment as compared with other variables such as remnants volume, size and number of metastases in lymph nodes. PATIENTS AND METHODS: 68 adult patients with differentiated thyroid cancer have been treated with radioiodine after thyroidectomy. All patients had metastatic lesions in lymph nodes accumulating (131)I. Activities administered amounted from 1000 to 6000 MBq. From 1 to 4 courses of radioiodine therapy were necessary for total ablation of metastases and remnants. Logistic function has been used to describe the probability of the total ablation of metastases and thyroid remnants after the first course. RESULTS: It was shown that the value of the first activity has the decisive influence on the treatment outcome. In patients included into analysis the outcome of the first course actually does not depend on level of radioiodine accumulation in pathological lymph nodes and effective half-time of (131)I excretion in remnants. CONCLUSION: As a result, in the case of metastatic lesions in lymph nodes accumulating radioiodine the first activity should not be less then 5000 MBq with the aim of minimizing the number of courses. Outcome of the treatment only slightly depends on such factors as histology, the number and the size of metastases in lymph nodes.