In the determination of therapeutic 131I doses, it takes several days to measure effective half life (EHL) of radioiodine in thyroid glands (Ordinary method). We suggested the new method to estimate EHL by a single radioiodine uptake measurement (INDEX method). We evaluated the outcome of 131I treatment with these two methods in outpatients with Graves' disease. Eighty outpatients were treated with INDEX method (Group I) and 108 outpatients with Ordinary method (Group O). At the 5-yr follow up, the incidence of hypothyroidism in Group I was 22.5%, subclinical hypothyroidism 8.8%, euthyroidism 30.0%, subclinical hyperthyroidism 13.7% and hyperthyroidism 25.0%. In Group O, 17.6% of the patients were hypothyroid, 16.7% subclinical hypothyroid, 30.5% euthyroid, 9.3% subclinical hyperthyroid and 25.9% hyperthyroid. There were no significant differences between these two methods. We conclude that INDEX method surpasses Ordinary method in timesaver and is equal in effectiveness.