BACKGROUND Ophthalmopathy caused by Graves' disease may first appear or worsen during or after treatment for hyperthyroidism. It is not known, however, whether choosing to treat hyperthyroidism with antithyroid drugs, iodine-131, or surgery affects the development or aggravation of Graves' ophthalmopathy. METHODS We studied 168 patients with hyperthyroidism caused by Graves' disease, stratified into two age groups--20 to 34 years (54 patients, group 1) and 35 to 55 years (114 patients, group 2). The patients in group 1 were randomly assigned to treatment with methimazole for 18 months or subtotal thyroidectomy, and those in group 2 to either of these two treatments or to iodine-131 therapy. All the patients received thyroxine to avert hypothyroidism, except those treated with iodine-131, who received thyroxine only if hypothyroidism developed. The duration of follow-up was at least 24 months. RESULTS Twenty-two patients (13 percent) had infiltrative Graves' ophthalmopathy at randomization. During follow-up, ophthalmopathy developed for the first time in 22 patients (13 percent) and worsened in 8 patients (5 percent). The frequency of the development or worsening of ophthalmopathy was similar among the patients in group 1 (medical therapy, 4 of 27 patients [15 percent]; and surgery, 3 of 27 patients [11 percent]). In group 2, ophthalmopathy developed or worsened in 4 of the 38 patients (10 percent) treated medically, 6 of the 37 patients (16 percent) treated surgically, and 13 of the 39 patients (33 percent) given iodine-131 (P = 0.02 for the comparison between the iodine-131 subgroup and the others combined). The risk of the development or worsening of ophthalmopathy increased as pretreatment serum triiodothyronine concentrations increased. CONCLUSIONS As compared with other forms of antithyroid therapy, iodine-131 is more likely to be followed by the development or exacerbation of Graves' ophthalmopathy.