The optimal method for determining iodine-131 treatment doses for Graves' hyperthyroidism is unknown, and techniques have varied from a fixed dose to more elaborate calculations based upon gland size, iodine uptake, and iodine turnover. Patients with Graves' hyperthyroidism (n = 88) who had not been previously treated with radioactive iodine were randomized to one of four dose calculation methods: low-fixed, 235 MBq; high-fixed, 350 MBq; low-adjusted, 2.96 MBq (80 micro Ci)/g thyroid adjusted for 24 h radioiodine uptake; and high-adjusted, 4.44 MBq (120 micro Ci)/g thyroid adjusted for 24 h radioiodine uptake. Subjects were followed for mean of 63 months (range, 10-94 months) for the following clinical outcomes: euthyroid without medication, hyperthyroid requiring further radioiodine, and hypothyroid requiring life-long L-T(4). Mean treatment doses were similar in the different outcome groups. We could not demonstrate any advantage to using an adjusted dose method. Survival analysis did not demonstrate any difference in the time to outcome between the fixed and adjusted dose methods. The use of a fixed dose method simplifies the approach to treatment with potential cost savings.