The serum concentration of sex-steroid-binding plasma protein (SBP) was measured with radioimmunoassay for 92 patients with Graves' disease, 36 males and 56 females, as well as 92 age- and sex-matched controls. In 24 male patients with hyperthyroidism, the mean SBP concentration was 6.4 +/- 0.2 (mean +/- SD) microgram/ml, whereas it was 2.3 +/- 0.2 micrograms/ml for the controls. The mean SBP concentration in 28 female patients with hyperthyroidism was 7.4 +/- 0.6 micrograms/ml, whereas it was 3.2 +/- 0.6 micrograms/ml for the controls. In Graves patients who were euthyroid following treatment, SBP levels were decreased near normal in both sexes. There was a significant correlation between T4 and SBP concentrations as well as between T3 and SBP concentrations in patients with Graves' disease in both sexes, with a better correlation in females. Increased SBP in untreated Graves' disease initiated to decline within a month following administration of antithyroid drugs, taking over 3 months to decline to the normal range.