Abstract We measured plasma levels of PGI 2 as 6-keto-prostaglandin F la (6-keto-PGFla) and thromboxane A 2 (TXA 2) as thromboxane B 2 (TXB 2) in patients with Graves' disease and in normal subjects. The levels of plasma 6-keto-PGFla were significantly elevated and correlated with those of serum T 4 and T 3, respectively, in hyperthyroid patients with Graves' disease. Significant reduction of 6-keto-PGF 1a levels was observed after antithyroid drug therapy. In contrast, the levels of plasma TXB 2 were significantly lower in untreated patients with Graves disease than in normal subjects. These data suggest that an elevation of plasma 6-keto-PGF 1a may play some additional role in pathophysiology of Graves disease.