[en] Aim: The micronucleus test in human peripheral lymphocytes is a frequently applied assay for biological dosimetry. In the present study, the dose-effect relationship was assessed between the radiation doses received by patients treated with iodine-131 for thyroid cancer and the frequency of micronuclei (Mn) in circulating lymphocytes of these patients. The aim of the study was to verify the sensitivity of the Mn assay to detect a putative accidental overexposure to radioactive iodine. Material and Methods: Frequencies of micronuclei in binucleated cells (BNC) obtained from venous blood were estimated in 29 patients with various forms of thyroid cancer who received between 166 MBq and 6.5 GBq of I-131. Blood was collected on day 4 following the administration of the iodine. The frequencies of Mn were compared to those detected in peripheral blood lymphocytes of 34 control donors, who were age- and sex-matched to the patients. Results: The mean Mn frequency in the patients' lymphocytes (32.8 per 1000 BNC) was significantly higher than in the lymphocytes of the control donors (19.4 per 1000 BNC; p<0,001, two-tailed t-test). In the patients, a strong inter-individual variability in Mn frequency was found. Due to this, only a weak correlation (r=0.19) was detected between the Mn frequency and the activity of I-131. In both groups of the donors, the Mn frequency increased with age. Conclusion: Although a significant difference in the Mn frequency was observed between the I-131-treated and control donors, the weak dose-effect relationship indicates that the Mn assay is not sensitive enough to detect an overexposure to this isotope, at least in the tested activity range