Two cases of asymptomatic adrenocortical insufficiency are reported. Both patients had a normal cortisol and increased ACTH. The cortisol response to ACTH was impaired, although not absent, in both cases. One case was associated with autoimmune polyglandular syndrome type II (Graves' disease and vitiligo), and the other was possibly associated with an early stage of Hashimoto's thyroiditis, suggesting autoimmune pathogenesis of their adrenocortical insufficiency. CT of the abdomen revealed unilateral enlargement of the adrenal glands in one case, but no enlargement of the adrenal glands in another case. Adrenal hypofunction seemed to be compensated for by increased trophic hormone (ACTH), as in subclinical hypothyroidism. However, prolonged ACTH stimulation increased urinary 17-OHCS in both cases, and normalized cortisol response to ACTH in one case. In both cases, the plasma renin activity (PRA) and plasma aldosterone concentration (PAC) were normal, and adrenal autoantibodies were negative, suggesting that neither negative adrenal autoantibodies nor normal PRA can exclude asymptomatic adrenocortical insufficiency. The results suggest that a rapid ACTH test should be performed in cases with increased ACTH, especially those associated with other autoimmune endocrine disorders.