38 patients with Graves' disease were treated at random with the glucocorticosteroid betamethasone or with placebo. The daily oral dose was 6.0 mg for the first 5 days, 4.5 mg for the following week, and then 3.0 mg. The serum free triiodothyronine (FT3) concentration decreased within 5 days, while the free thyroxine (FT4) level was reduced first after 3 weeks of betamethasone treatment. The suppressed serum thyrotropin concentration did not change. In the placebo group no significant constant alterations were found in any of the variables studied. The results corroborate that betamethasone decreases FT3 and to a less degree also FT4, which earlier has been indicated by indirect methods, although the mechanisms behind the changes remain to be clarified. Since FT3 is more readily available for the metabolic effects in tissues the rapid striking fall in its concentration is an argument for glucocorticoid treatment in selected patients with severe hyperthyroidism of Graves' disease.