The effect of thyroid hormone excess on glucose tolerance as well as insulin secretion and its peripheral action has been a matter of debate for many years. Thyrotoxicosis caused by Graves' disease is associated in some patients with impaired glucose tolerance and insulin resistance. The objective of the present study was to investigate if the insulin sensitivity, assessed by the euglycemic insulin clamp technique, increase after correction of hyperthyroidism due to Graves' disease. After four months of medical treatment, patients became euthyroid and insulin sensitivity increased significantly from 3.47 to 6.39 mg/kg/min; therefore it was concluded that insulin resistance could be improved after successful treatment of hyperthyroidism. The precise mechanism underlying the effect of thyroid hormone excess on insulin sensitivity remains to be elucidated.