Despite extensive use of iodine-131 ((131)I) treatment for Graves' hyperthyroidism, the optimal regimen of pretreatment with antithyroid drugs is still a matter of discussion. Our aim was to evaluate the success of (131)I treatment in patients with Graves' disease without and with pretreatment with methimazole (MMI). In a prospective randomized study 156 patients with Graves' disease were treated with fixed activity of 550 MBq (131)I. First group of 59 patients received only (131)I. The second group of 50 patients received MMI which was stopped seven days before (131)I. The third group of 47 patients received MMI until (131)I application. Patients were followed clinically and biochemically 1, 3, 6 and 12 months after (131)I treatment. Absorbed dose of (131)I and thyroid volume were measured in each patient. Our result showed that (131)I treatment success after twelve months was equally effective in the first and second group (96.6% and 96%, respectively), while in the third group, success was significantly lower (63.8%). Accordingly, the absorbed dose of (131)I was significantly higher in the first and in second group (144±104 Gy and 164±107 Gy, respectively), and lower in the third group (105±58 Gy). Thyroid volume gradually decreased without any significant difference between the three groups. In conclusion, our study provides evidence that application of (131)I is equally effective in the nonpretreated with MMI group and in the group discontinuing MMI one week before (131)I treatment, and it is more effective in these two groups as compared to the group in which pretreatment with MMI was administered till the day of (131)I application.