We describe a woman who developed adrenal insufficiency after removal of an apparently nonfunctional adrenal adenoma. She displayed no stigmata of Cushing's syndrome and had normal plasma and urinary cortisol levels. A second patient without clinical findings of Cushing's syndrome also had normal basal steroid levels. This patient displayed partial suppressibility with dexamethasone, had low-normal levels of serum corticotropin, and excreted a low concentration of urinary 17-ketosteroids. She also developed mild adrenal insufficiency after the operation. We believe the adrenal adenomas in these patients secreted enough cortisol to suppress the contralateral adrenal gland but not enough hormone to elevate basal steroid levels. Therefore, we suggest that all patients with adrenal masses be studied with the overnight dexamethasone suppression test rather than basal steroid hormone measurements to detect low levels of autonomous cortisol secretion. In addition, patients with adrenal masses that are not removed surgically should have serial adrenal function tests performed.