A retrospective analysis was done of the records of 454 patients who received their first I-131 treatment for Graves' disease during six periods covering 1951 to 1978. In the earliest group, 3% of patients were hypothyroid 3 mo after I-131 use, and 40% were hypothyroid at 1 yr. In the most recent group, 36% of patients were hypothyroid at 3 mo and 91% were myxedematous at 1 yr. Although no obvious trends were noted, whether in the number of patients pretreated with thionamide drugs, in the mean 24-hr I-131 uptake, or in the calculated dose of I-131 (muCi/estimated gram of thyroid tissue) during the years of the study, the initial mean dose of I-131 administered increased from 8.1 mCi in the earliest group to 13.8 mCi in the latest group. Concurrently, estimates of gland size increased from a mean of 26 g in the first group to 43 g in the last. If, in patients with Graves' disease, the thyroid gland size did not truly increase during the years of the study, the increasing occurrence of early hypothyroidism seen after I-131 use may reflect the conscious or unconscious decision to use larger doses of I-131 calculated on the basis of inflated estimates of thyroid gland weight.