Little is known about the factors that cause exacerbations of autoimmune thyroid dysfunction. One possibility is an alteration in adrenocortical function, since glucocorticoids are known to alter both pituitary-thyroid and immunologic function. We encountered three patients in whom overt autoimmune thyroid disease developed after unilateral adrenalectomy for Cushing's syndrome due to an adrenocortical adenoma. We compared the postoperative changes in thyroid function in these patients with those in 21 other patients with Cushing's syndrome who underwent the same treatment. After unilateral adrenalectomy, one of the three patients had transient hyperthyroidism and a low thyroid uptake of 131I, indicative of silent thyroiditis. After the same surgical procedure, the second patient had hypothyroidism, where-as the third patient had transient hyperthyroidism at first, and hypothyroidism then gradually developed. All three patients had serum antithyroid antibodies, the titers of which increased after surgery. In the remaining 21 patients (only 2 of whom had antithyroid antibodies initially), the serum concentrations of thyroxine, triiodothyronine, and thyroxine-binding globulin and the secretion of thyroid-stimulating hormone increased after surgery from values that were low or near the lower limit of normal to values still well within the normal range. None of these patients had clinically evident thyroid disease or increased antithyroid-antibody titers. We conclude that reductions in the secretion of glucocorticoid may exacerbate subclinical autoimmune thyroid disease. Patients with Cushing's syndrome due to adrenocortical adenoma who have thyroid antibodies should be followed closely after treatment, because thyroid dysfunction may develop.