257 patients have been reviewed 1-5 years (mean 3 years 2 months) after receiving one of five dose regimes of 125I for thyrotoxicosis. The cumulative incidence of hypothyroidism was 34% and of persistent thyrotoxicosis 17%. The group receiving doses between 351 and 500 muCi/g had the highest proportion of euthyroid patients (65%) with the lowest requirement for repeat therapy (46%). In the euthyroid patients, increasing dose of 125I was associated with progressive decline in mean thyroxine (T4) level and free thyroxine index (FTI) within the respective normal ranges, and increase in mean thyroid stimulating hormone (TSH) level to above the normal range. Euthyroid patients with elevated TSH levels had significantly lower T4 and FTI values compared with those with normal TSH, and showed a 3-4-fold increased rate of development of hypothyroidism over 1 year. Euthyroid patients with elevated T3 levels remained euthyroid during the subsequent year and mean T3 levels declined significantly, suggesting that abnormally elevated T3 levels after 125I do not generally indicate impending relapse of thyrotoxicosis. It is concluded that the potential admantages of 125I therapy for thyrotoxicosis in reducing the incidence of hypothyroidism have not been realized in practice.