OBJECTIVE To evaluate the results of treatment of Graves' disease with 131I at low calculated doses. METHODS A total of 333 patients with Graves' disease were treated with low calculated doses of 131I on the basis of thyroid size and 131I uptake (mean doses = 6.6 +/- 1.9 mCi). The mean follow-up was 24.4 months (range 12-145). RESULTS The accumulated likelihood of hypothyroidism at 145 months of follow-up was 89.8% (Kaplan-Meier method) and relapse 26%. Pretreatment levels of T3 above 9 nmol/l were associated with a lower incidence of hypothyroidism (p = 0.049, Mantel-Cox method). Pretreatment levels of T4 above 300 nmol/l were associated with a higher risk of therapy failure (odds ratio 3.27; 95% confidence interval = 1.3-8.2, Cox method). Age, sex, previous therapy with anti-thyroid agents or surgery, 131I uptake, initial and total dose of 131I, number of administered doses and development of transient hypothyroidism were not predictive of the evolution of thyroid function. CONCLUSIONS Therapy of hyperthyroidism (Graves' disease) with low calculated doses of 131I shows a high incidence of hypothyroidism, also increasing the cost involved in estimating the dose and long-term follow-up.