Adrenal function was assessed in 52 euthyroid patients (48 of whom were taking thyroxine replacement therapy) in whom autoimmune thyroiditis had been diagnosed between one and a half and 17 years previously. There was no clinical evidence of adrenal hypofunction. Circadian plasma cortisol concentration was normal in 49 patients and there was no evidence of impaired adrenal reserve as indicated by the response to tetracosactrin (Synacthen) in any patient. Autoantibodies to adrenal cortex were present in only one patient. These findings contrast with the high incidence of impaired thyroid function reported in idiopathic Addison's disease.