This report details the 26- and 36-yr outcomes of 116 patients under the age of 20 yr with Graves' disease who were treated with radioiodine between 1953 and 1973. Contacted by telephone and mail in 1991-1992, 107 of them supplied personal historical data, and their physicians furnished interval histories, physical examinations, and laboratory data. This was repeated in 2001-2002, with 98 of them being contacted. At the time of treatment, the patients' ages ranged between 3 yr, 7 months and 19 yr, 9 months. Six were less than 6 yr of age, 11 were between 6 and 11 yr, 45 were between 11 and 15 yr, and 45 were between 16 and 19 yr. The average length of follow-up in 1991-1992 was 26.1 yr; that in 2001-2002 was 36.2 yr. None of the patients developed cancer of the thyroid or leukemia. Early on, when the objective of treatment was euthyroidism, the dose of radioiodine was low, and retreatment was frequently needed. Later, the doses used were increased. Over time, all but two patients became hypothyroid. Pregnancies did not result in an unusual number of congenital anomalies or spontaneous abortions. Treating young people with Graves' disease with radioiodine is safe and effective over the long term.