Background: Radio-iodine is a definite therapy for Graves disease hyperthyroidism. However, the optimal dosage is still debatable. Aim: To assess the effects of different radioiodine doses on thyroid function and complications in patients with hyperthyroidism. Material and methods: A retrospective analysis of 139 patients with hyperthyroidism, treated with ratioiodine between 1988 and 1998. Radio iodine dose used was classified as low (<10 mCi), intermediate (10-14.9 mCi) or high ( > or = 15 mCi). Results: Thirty five patients were treated with low doses, 33 with intermediate doses and 71 with high doses. There were no differences between these patients in age, disease severity, frequency of post treatment euthyroidism or complications. Patients treated with low doses had a higher frequency of persistent hyperthyroidism than patients treated with high doses (25.7 and 4.2% respectively, p<0,001). Likewise, the frequency of subsequent hypothyroidism was 60% in patients treated with low doses and 84.5% of those with high doses, in whom it also appeared earlier. Associated complications were clinically irrelevant. In seven patients, Graves ophtalmopathy progressed after treatment, but this progression was not associated with the dose used. Conclusions: Radio iodine in high doses is useful, safe and effective for the treatment of Graves hyperthyroidism. (Rev Méd Chile 2000; 128: 499-507).