I-131 was administered to 298 patients with thyroid cancer, and there has been a follow-up of at least two years. Follow-up periods were: 2.5 to 30 years (median 14.5) in living patients, 2.5 to 15 years (median 5.5) in patients dead of tumour greater than or equal to 2 years after first treatment and 2.5 to 23 years (median nine) in patients dead without tumour. Person-years at risk were (total applied activity of I-131): 1119 (3 to 21 GBq), 1477 (22 to 65 GBq), 521 (61 to 170 GBq). 33 subsequent malignancies in 31 patients were observed, compared to an expected number of 17. The relative risk of subsequent malignancies is therefore 1.94 with a 95% confidence interval of 1.15 to 3.05. This increase in the incidence of subsequent malignancies after I-131 treatment is largely due to the significantly increased incidence of leukemia and bladder cancer. Estimated radiation doses to the bone marrow in the patients with leukemia were 301 cGy to 792 cGy and the doses to the bladder in patients with bladder cancer were 2250 cGy to 10, 350 cGy. After a total activity of less than 37 GBq I-131, no cases of bladder cancer or leukemia were observed. The observed number of subsequent malignancies are compared with the expected number according to several dose-effect estimations.