The influence of betamethasone on iodothyronines and their binding proteins in serum was studied in 18 patients with Graves' disease. Betamethasone, 6 mg daily, was given orally for 5 days. Serum triiodothyronine (T3) concentration decreased, reverse triiodothyronine increased, while thyroxine (T4) and thyroid-stimulating hormone were unchanged. Thyroxine-binding globulin (TBG) decreased and thyroxine-binding prealbumin increased. In accordance with the decrease in TBG, the in vitro T3 uptake increased and out of the indirect measures of the free thyroid hormones, the free T4 index increased and the free T3 index decreased. We suggest that the decrease in serum T3 during betamethasone administration supports the view of glucocorticoids being beneficial as additional treatment in some patients with thyrotoxic crisis.