The primary objective of the present work was to study the clinical course of endocrine ophthalmopathy (EOP) following radioiodine therapy (RIT) of Graves' disease (GD) and depending on its effect (development of post-radiation hypothyroidism). The secondary objective was to determine risk factors of EOP progression after radioiodine therapy. This prospective study included 38 patients (76 eyes) allocated to two groups. The patients of group 1 (n=19/38 eyes) presented with thyrotoxicosis at each visit and continued to use thyrostatic agents; those in group 2 (n=19/38 eyes) had hypothyroidism at its early stages (3 and 6 months) and were given substitution therapy with levothyroxin. The development of post-radiation hypothyroidism was shown to strongly influence the clinical course of EOP. In the patients of group 1, EOP remained active throughout the entire observation period (12 months) in the absence of appreciable variations of its integral severity index. In group 2, the same index decreased significantly, but active forms of EOP could be detected by the time of onset of hypothyroidism (6 months) (p=0.0000). After 12 months, the level of anti-TSH receptor antibodies in the patients of group 1 was significantly higher than in those of group 2 (10.8±8.3 and 2.9±2.0 respectively, p=0.0003). The regression rate of EOP symptoms following radioiodine therapy (RIT) of Graves' disease was a function of the efficacy of thyroid 131I radioablation. It is concluded that persistence of anti-TSH receptor antibodies was responsible for the deterioration of the clinical picture of endocrine ophthalmopathy after radioiodine therapy.