Radioiodine (131I) treatment of well-differentiated thyroid carcinoma is a well-evaluated therapeutic model for nuclear medicine which has never been equaled by subsequent developments. It is still a unique method of treating cancer. The treatment of thyroid cancer begins with a systematic approach to the most common first symptom or sign; a neck mass. Data have accumulated to show that well-differentiated thyroid cancer does kill commonly enough to warrant aggressive treatment, even in young individuals. There is also evidence that the more complete the thyroidectomy, the lower the death and recurrence rate of the thyroid cancer, and the more effective the use of 131I in both detecting and treating metastases. There are now considerable data demonstrating that 131I after surgery decreases both the recurrence rate and death rate from well-differentiated thyroid cancer. After uptake is "ablated", there is a 1%--2% recurrence rate in patients with the most extensive disease at the time of the initial treatment. This recurrence is effectively retreated with another dose of 131I. Surgery and 131I should be used as long as they are effective before resorting to teletherapy. There are now considerable data to show that the morbidity of surgical and 131I treatment is reasonable in contrast to the recurrence and death rate from nonaggressively treated well-differentiated thyroid carcinoma. Serious consideration should be given to using a low iodine diet before treatment with radioiodine.