In an attempt to study the effect of adrenal steroids on plasma thyroid hormone concentration in patients with Hashimoto's thyroiditis, 2 mg dexamethasone was administered for 2-4 weeks to 9 patients with normal plasma TSH, 3 patients with compensatory increase of plasma TSH, and 10 patients with a marked elevation of plasma TSH. Dexamethasone depressed plasma T4, T3, and TSH and reduced thyroid size in nine euthyroid patients, whereas a similar treatment did not affect plasma T3 and T4, but reduced plasma TSH and thyroid size in three patients with compensatory increase of plasma TSH. In contrast, dexamethasone elevated plasma T3 and T4 in 10 hypothyroid patients with a marked elevation of plasma TSH. The increase of T3 was more than that of T4, suggesting preferential secretion of T3. Plasma TSH was reduced, but was still above normal after administration of dexamethasone. It is suggested that the intrathyroidal autoimmune processes inhibiting thyroid hormone synthesis are significant in patients with more severe Hashimoto's thyroiditis, and that dexamethasone stimulated hormone synthesis by depressing the autoimmune processes.