A multicentre study was undertaken to assess the cytogenetic damage to peripheral blood lymphocytes in 31 patients treated with 131I for thyrotoxicosis using the cytokinesis-blocked micronucleus assay. The results were compared to those for eight thyroid carcinoma patients using the same method. For each patient, blood samples were taken immediately before and 1 week after iodine administration. The first blood sample was divided into three fractions and each fraction was subsequently irradiated in vitro with 0, 0.5 and 1 Gy 60Co gamma rays, respectively. After blood culture for 70 h, cells were harvested, stained with Romanowsky-Giemsa and the micronuclei scored in 1000 binucleated cells. For both patient groups, a linear-quadratic dose-response curve was fitted through the data set of the first blood sample by a least squares analysis. The mean increase in micronuclei after 131I therapy (second blood sample) was fitted to this curve and the mean equivalent total body dose (ETBD) calculated. Surprisingly, in view of the large difference in administered activity between thyroid carcinoma patients and thyrotoxicosis patients, the increase in micronuclei after therapy (mean +/- S.D.: 32 +/- 30 and 32 +/- 23, respectively) and the equivalent total body dose (0.34 and 0.32 Gy, respectively) were not significantly different (P > 0.1). The small number of micronuclei induced by 131I therapy (32 +/- 29), compared with external beam radiotherapy for Hodgkin's disease (640 +/- 381) or cervix carcinoma (298 +/- 76) [1], gave a cancer mortality estimate of less than 1%. This also explains why late detrimental effects in patients after 131I treatment have not been reported in the literature.